diff --git a/README.md b/README.md index 5aa45cf..c7e5887 100644 --- a/README.md +++ b/README.md @@ -522,11 +522,17 @@ deterministic per-username dummy challenge, so probing the daemon cannot enumerate users. Store lines are generated with `fastsync-server --hash-credentials ` (see `RSYNC_COMPAT.md`); redirect that output to an owner-only (mode 0600) file, and note that legacy -`user:SHA256HEX` stores are rejected. Two residuals are accepted: the dummy salt -is stable within one daemon lifetime but changes across restarts, so a -restart-gated enumeration channel remains (persisting a dummy key is out of -scope); and the store iteration count is observable pre-auth by design, since -the miss path must match a hit. +`user:SHA256HEX` stores are rejected. FastSync also maintains an owner-only +(mode 0600) `.dummykey` sidecar next to the store: it holds the store-wide +dummy key, is auto-created on first load, and must be preserved across daemon +restarts so the dummy challenge for an unknown user stays stable (the key is +never regenerated while the sidecar exists). If the sidecar cannot be created +(process-substitution/FIFO store path such as `/dev/fd/N`, a read-only +filesystem, or a missing directory), the daemon logs a warning and uses a +transient key, so the cross-restart guarantee does not hold for those +deployments. One residual is accepted: the store +iteration count is observable pre-auth by design, since the miss path must match +a hit. An `auth users` module accepts credentials only when one of two conditions holds: (a) the connection is an encrypted, verified TLS connection whose client diff --git a/RSYNC_COMPAT.md b/RSYNC_COMPAT.md index 8b96e8a..191cb49 100644 --- a/RSYNC_COMPAT.md +++ b/RSYNC_COMPAT.md @@ -639,8 +639,8 @@ now transmits targets (the prior behavior was broken/partial); its status moved - **Module selection & confinement:** the client requests a module with an rsync-style `host::module[/path]` destination. The module name crosses the wire as a trailing string on the config frame (bumping `PROTOCOL_VERSION` 2.14.0 → 2.15.0; the bump is required because the config-frame layout changed and the strict same-version handshake is what prevents a peer from desynchronizing on the new trailing field). The daemon looks the module up in ITS OWN config and uses the module's `path` as the authorized root through the exact same `configure_authorization` confinement the standalone server applies to `--destination-root` (`file_open_secure_parent`, `has_path_traversal`, `path_is_within`); the client never supplies the root, every client-chosen-ownership/super-user request is refused unless the module declares `client owner = yes` (the daemon's per-module opt-in, see below), and the operator `--no-super` veto forces super-user activities off for every daemon connection. The client's `/path` part is relative inside the module and is rejected if absolute or if it contains `..`. Unknown modules are refused before any data moves (the run fails cleanly at the config handshake). An absolute destination and a module request against a non-daemon server are also refused. - **`client owner` (client-chosen-ownership opt-in):** by default a daemon module refuses every request that would let the client pick an owner or ask for super-user activities — `--numeric-ids`, `--chown`, `--usermap`/`--groupmap`, `--fake-super`, `--copy-as`, and an explicit `--super` — at the config handshake (before `STATUS_OK`), because a daemon has no per-module opt-in for client-chosen ownership and any anonymous client could otherwise force arbitrary owner ids inside the module root. `client owner = yes` opts a single module in, allowing those requests within that module's root (the standalone listener and the SSH `--stdio` server always honor them for their single operator-authorized root). Without the opt-in the daemon also forces super-user **device** activity off for that connection — char/block device-node creation (`--devices`) and `--write-devices` — even under the default `AUTO` mode, so a non-opted module can never be made to `mknod` or write a raw device; those entries are skipped (not refused) so an ordinary `-a` push still succeeds without device nodes. The opt-in does **not** lift the privilege requirement: `--copy-as` still needs a root receiver, and the operator `--no-super` veto still forces super-user activities off for every connection. The daemon logs a prominent startup warning for each `client owner = yes` module so the operator's deliberate choice is visible. - **`read only` safe default:** every network transfer FastSync currently supports is a push that writes under the module root, so a `read only` module refuses the connection (clear server log "module is read only"; the client exits non-zero, nothing is transferred). A future pull/list operation can be opened up when it exists; the knob is already stored. -- **`auth users` (A7 SCRAM-SHA-256 authentication):** a module that declares `auth users` requires the client to present credentials. The config frame carries ONLY the username; the daemon answers an auth-required module with `STATUS_AUTH_CHALLENGE` (PBKDF2 iteration count, 16-byte salt, 32-byte server nonce), the client answers with `STATUS_AUTH_RESPONSE` (fresh 32-byte client nonce + a 32-byte ClientProof), and the daemon accepts only when the proof verifies **and** the username is **on the module's `auth users` list** and has a store entry, replying `STATUS_AUTH_OK` with a 32-byte ServerSignature the client verifies before proceeding. Verification is constant-time over fixed 32-byte keys (the compare runs even for a miss), username membership uses a constant-time full-length scan, and an unknown/off-list user still receives a challenge and runs the same math against a dummy verifier: a deterministic per-username salt (`HMAC-SHA256(store dummy key, username)`), the store-wide uniform iteration count and dummy keys. Re-probing the same unknown username therefore yields an identical salt and iteration count while a different username yields a different salt, so there is no user-enumeration or timing oracle. The daemon logs the username but **never the password, proof or keys**. A module WITHOUT `auth users` stays open (legitimate rsync configuration); credentials sent to such a module are ignored. Read-only is orthogonal: even a correctly authenticated push to a `read only` module is still refused (all FastSync network transfers write). Fail-closed policy: a daemon whose config declares `auth users` on any module refuses to start unless a credential store was given (`--password-file` and/or `--early-input`); a missing or empty store is never silently treated as "open". A failed handshake (missing credentials, unknown/off-list user, wrong proof or malformed data) yields a single generic `STATUS_AUTH_FAILED` and the daemon closes before any data moves. Two residuals are accepted: the dummy salt is stable within one daemon lifetime but changes across restarts, leaving a restart-gated enumeration channel (persisting the dummy key is out of scope); and the store iteration count is observable pre-auth by design, since the miss path must match a hit. **Transport policy (hardening A7-3/S1):** an auth-required module accepts credentials only when either (a) the connection is an encrypted, verified TLS connection whose client certificate matches `--client-cn`, or (b) the connection is plaintext from a loopback TCP peer **and** the operator explicitly passed `--allow-unauthenticated`. A remote plaintext peer, and a loopback plaintext peer without that flag, are refused at the config gate before any challenge is sent; `--allow-unauthenticated` never permits remote plaintext auth (remote peers still require verified TLS). Daemon modules are a `--daemon`-only feature — the SSH `--stdio` path never loads a daemon config and is not an auth transport for them. Because the loopback allowance trusts whichever peer the kernel reports as `127.0.0.1`, it assumes nothing relays remote connections to the daemon: a local TCP forwarder or TLS-terminating proxy in front of an auth-module listener makes remote clients appear as loopback and bypasses the mutual-TLS identity check, so do not front an auth-module listener with such a relay. -- **Credential store format:** server `--password-file`/`--early-input` files are line-based `user:$fastsync$1$pbkdf2-sha256$$$$`, one per line (standard base64; 16-byte salt, 32-byte keys; `iters` in `[100000, 10000000]`, default 600000). Every entry in the resulting store must agree on `iters` (a store whose entries disagree, or where a layered `--early-input` disagrees with `--password-file`, is rejected). Generate lines with `fastsync-server --hash-credentials FILE [--iterations N]`; the emitted lines are secret material, so redirect them to an owner-only (mode 0600) file (the tool warns on stderr if stdout is a group/other-accessible regular file). Blank lines and lines starting with `#`/`;` are comments; the parser is strict (a malformed line fails the whole load, so a typo can never let a different set of users in). **The legacy `user:SHA256HEX` form is hard-rejected** with an actionable "legacy" error; there is no auto-upgrade, so a replayable bearer digest can never be loaded by a 2.19.0 daemon. The client `--password-file` holds `user:password` on its first meaningful line (the literal password, used only for the handshake then burned); keep both files readable only by their owner (mode 0600). Per-username wire length is bounded (256 chars) and every decoded salt/key length is validated. +- **`auth users` (A7 SCRAM-SHA-256 authentication):** a module that declares `auth users` requires the client to present credentials. The config frame carries ONLY the username; the daemon answers an auth-required module with `STATUS_AUTH_CHALLENGE` (PBKDF2 iteration count, 16-byte salt, 32-byte server nonce), the client answers with `STATUS_AUTH_RESPONSE` (fresh 32-byte client nonce + a 32-byte ClientProof), and the daemon accepts only when the proof verifies **and** the username is **on the module's `auth users` list** and has a store entry, replying `STATUS_AUTH_OK` with a 32-byte ServerSignature the client verifies before proceeding. Verification is constant-time over fixed 32-byte keys (the compare runs even for a miss), username membership uses a constant-time full-length scan, and an unknown/off-list user still receives a challenge and runs the same math against a dummy verifier: a deterministic per-username salt (`HMAC-SHA256(store dummy key, username)`), the store-wide uniform iteration count and dummy keys. Re-probing the same unknown username therefore yields an identical salt and iteration count while a different username yields a different salt, so there is no user-enumeration or timing oracle. The daemon logs the username but **never the password, proof or keys**. A module WITHOUT `auth users` stays open (legitimate rsync configuration); credentials sent to such a module are ignored. Read-only is orthogonal: even a correctly authenticated push to a `read only` module is still refused (all FastSync network transfers write). Fail-closed policy: a daemon whose config declares `auth users` on any module refuses to start unless a credential store was given (`--password-file` and/or `--early-input`); a missing or empty store is never silently treated as "open". A failed handshake (missing credentials, unknown/off-list user, wrong proof or malformed data) yields a single generic `STATUS_AUTH_FAILED` and the daemon closes before any data moves. The dummy key is persisted in an owner-only `.dummykey` sidecar (auto-created on first load, mode 0600) so the dummy salt stays stable across daemon restarts, closing the restart-gated enumeration channel. The sidecar must be preserved across restarts for that guarantee; if it cannot be created (a process-substitution/FIFO store path such as `/dev/fd/N`, a read-only filesystem, or a missing directory), the daemon logs a warning and uses a transient per-run key, so unknown-user challenges change across restarts and the cross-restart guarantee does not hold for that deployment. One residual is accepted: the store iteration count is observable pre-auth by design, since the miss path must match a hit. **Transport policy (hardening A7-3/S1):** an auth-required module accepts credentials only when either (a) the connection is an encrypted, verified TLS connection whose client certificate matches `--client-cn`, or (b) the connection is plaintext from a loopback TCP peer **and** the operator explicitly passed `--allow-unauthenticated`. A remote plaintext peer, and a loopback plaintext peer without that flag, are refused at the config gate before any challenge is sent; `--allow-unauthenticated` never permits remote plaintext auth (remote peers still require verified TLS). Daemon modules are a `--daemon`-only feature — the SSH `--stdio` path never loads a daemon config and is not an auth transport for them. Because the loopback allowance trusts whichever peer the kernel reports as `127.0.0.1`, it assumes nothing relays remote connections to the daemon: a local TCP forwarder or TLS-terminating proxy in front of an auth-module listener makes remote clients appear as loopback and bypasses the mutual-TLS identity check, so do not front an auth-module listener with such a relay. +- **Credential store format:** server `--password-file`/`--early-input` files are line-based `user:$fastsync$1$pbkdf2-sha256$$$$`, one per line (standard base64; 16-byte salt, 32-byte keys; `iters` in `[100000, 10000000]`, default 600000). Every entry in the resulting store must agree on `iters` (a store whose entries disagree, or where a layered `--early-input` disagrees with `--password-file`, is rejected). Generate lines with `fastsync-server --hash-credentials FILE [--iterations N]`; the emitted lines are secret material, so redirect them to an owner-only (mode 0600) file (the tool warns on stderr if stdout is a group/other-accessible regular file). Blank lines and lines starting with `#`/`;` are comments; the parser is strict (a malformed line fails the whole load, so a typo can never let a different set of users in). **The legacy `user:SHA256HEX` form is hard-rejected** with an actionable "legacy" error; there is no auto-upgrade, so a replayable bearer digest can never be loaded by a 2.19.0 daemon. The client `--password-file` holds `user:password` on its first meaningful line (the literal password, used only for the handshake then burned); keep both files readable only by their owner (mode 0600). Per-username wire length is bounded (256 chars) and every decoded salt/key length is validated. Loading the store also maintains an owner-only `.dummykey` sidecar (auto-created, mode 0600, exactly 32 bytes) holding the store-wide dummy key that shapes unknown-user challenges; persist it across daemon restarts so those challenges stay stable, and treat a sidecar with the wrong owner, a mode other than exactly 0600, the wrong size or the wrong type as a fatal load error (fail closed). If the sidecar cannot be created (e.g. a process-substitution store path such as `/dev/fd/N`, a read-only filesystem, or a missing directory), the daemon logs a warning and uses a transient per-run key, so the cross-restart stability guarantee does not hold there. - **Plaintext caveat:** an auth-required module is refused, **before any challenge is sent**, unless the connection is encrypted and verified TLS whose client certificate matches the server's `--client-cn`, or it is plaintext from a loopback TCP peer **and** the operator passed `--allow-unauthenticated`. A remote plaintext peer, and a loopback plaintext peer without that flag, never receive a challenge, and `--allow-unauthenticated` never permits remote plaintext auth (remote peers still require verified TLS). On the loopback plaintext transport that remains permitted, a local sniffer could still read the challenge and response and mount an **offline dictionary attack** against a weak password, so use `--tls` for any real deployment. `--client-cn` matches the certificate CN only (not a subjectAltName), which is acceptable for a private CA. Clients sending daemon credentials with `--password-file` to a non-loopback daemon must use `--tls`; the client rejects such a destination before any network I/O. Unlike the old challenge-less exchange there is **no replay**: the proof is bound to the fresh per-connection server nonce, so a captured `STATUS_AUTH_RESPONSE` cannot be reused on another connection (an integration test proxies the daemon and proves this). TLS client-CN (`--client-cn`) is an independent transport identity check and composes with password auth: both may be required on the same connection. - **Wire/protocol:** the config-frame auth block is now `[int present][str_redacted username]` (the old digest field is gone), and the frame stream gains the challenge/response (`STATUS_AUTH_CHALLENGE` → `STATUS_AUTH_RESPONSE` → `STATUS_AUTH_OK`/`STATUS_AUTH_FAILED`) between the config frame and the `STATUS_OK` ack. Both are wire-layout changes, so `PROTOCOL_VERSION` is bumped **2.18.0 → 2.19.0** (see the A7 note in `src/shared/config.h`); the strict same-version handshake keeps a 2.19 client and a 2.18 server from desynchronizing. - **Client side:** `host::module/path` selects the TCP transport and connects to `--server-port`; `host:path` stays the SSH transport; plain paths stay local TCP. The daemon username comes from `--password-file` (first `user:password` line), and `--password-file` without a `host::module/path` destination is a client error (fail fast). A `user@host::module` form is rejected with a pointer to `--password-file`. The client's plaintext password is wiped from memory (`config_burn_auth`) at transfer teardown. diff --git a/src/shared/credentials.c b/src/shared/credentials.c index 5971a26..f1a0c98 100644 --- a/src/shared/credentials.c +++ b/src/shared/credentials.c @@ -1,4 +1,5 @@ #include "credentials.h" +#include "log.h" #include "utils.h" #include #include @@ -35,16 +36,22 @@ struct CredentialStore { * challenged with the same count as a hit and the count itself never leaks * membership. Unused (0) for an empty store. */ uint32_t iters; - /* Random secret generated once at load. The dummy salt handed out for an - * unknown/off-list user is HMAC-SHA256(dummy_key, username)[:SALT_LEN], so - * repeated probes of the same username always see an identical challenge - * while different usernames differ -- with no fresh-random tell. */ + /* Store-wide secret loaded from (or created in) the exact-mode-0600 + * `.dummykey` sidecar, so it also survives a daemon restart. The + * dummy salt handed out for an unknown/off-list user is + * HMAC-SHA256(dummy_key, username)[:SALT_LEN], so repeated probes of the same + * username always see an identical challenge while different usernames differ + * -- with no fresh-random tell, and cross-restart stability hides the + * restart-gated enumeration oracle. */ uint8_t dummy_key[CREDENTIAL_KEY_LEN]; }; /* Exact marker prefix of the new store verifier field. */ #define CREDENTIAL_STORE_PREFIX "$fastsync$1$pbkdf2-sha256$" #define CREDENTIAL_AUTH_PREFIX "FastSync-Auth-v1" +/* Exact-mode-0600 sidecar holding the persistent store-wide dummy key, placed + * next to the credential store (`.dummykey`). */ +#define CREDENTIAL_DUMMY_KEY_SUFFIX ".dummykey" /* Fixed dummy keys used when a user is unknown or off the module's list. They * can never authenticate because acceptance additionally requires found=true. */ @@ -66,7 +73,10 @@ static bool is_comment_char(char c) { /* Open a --password-file / --early-input after verifying the EXACT inode we * will read: it must be owned by the effective user and grant no group/other - * permission bit (mode 0600), mirroring the TLS private-key check. We open by + * permission bit (so 0600 and stricter modes such as 0400 are accepted), + * mirroring the TLS private-key check. This only rejects group/other bits, + * deliberately unlike the dummy-key sidecar which requires EXACT mode 0600. We + * open by * path and then fstat the resulting fd (rather than stat()ing the path first * and reopening it), so the permission decision is made on the same inode that * is read and cannot be raced by swapping the path between check and open. @@ -583,6 +593,311 @@ static CredentialStore* load_store_file(const char* path, char* err, size_t err_ return store; } +/* Validate and read an already-open `.dummykey` sidecar. Fails closed on + * anything that is not an exact-mode-0600 regular file of exactly + * CREDENTIAL_KEY_LEN bytes, so a loosened, swapped or truncated file can never + * silently change the dummy challenge. */ +static bool read_dummy_key_fd(int fd, const char* path, uint8_t out[CREDENTIAL_KEY_LEN], char* err, + size_t err_size) { + struct stat st; + if (fstat(fd, &st) != 0) { + set_error(err, err_size, "cannot stat dummy key file '%s': %s", path, strerror(errno)); + return false; + } + if (!S_ISREG(st.st_mode) || st.st_uid != geteuid() || (st.st_mode & 07777) != 0600 || + st.st_size != (off_t)CREDENTIAL_KEY_LEN) { + set_error(err, err_size, + "refusing to read dummy key file '%s': it must be an owned regular file with exact " + "mode 0600 and exactly %d bytes", + path, CREDENTIAL_KEY_LEN); + return false; + } + size_t got = 0; + while (got < CREDENTIAL_KEY_LEN) { + ssize_t n = read(fd, out + got, CREDENTIAL_KEY_LEN - got); + if (n < 0) { + if (errno == EINTR) + continue; + set_error(err, err_size, "cannot read dummy key file '%s': %s", path, strerror(errno)); + return false; + } + if (n == 0) + break; + got += (size_t)n; + } + if (got != CREDENTIAL_KEY_LEN) { + set_error(err, err_size, "dummy key file '%s' is truncated", path); + return false; + } + return true; +} + +/* fsync the directory containing `path` (best effort). After publishing the + * sidecar with link(2), syncing the directory makes the new name durable so a + * crash cannot leave a restart without the key it just started using. */ +static void fsync_containing_dir(const char* path) { + char* dir = str_dup(path); + if (!dir) + return; + char* slash = strrchr(dir, '/'); + if (!slash) { + free(dir); + dir = str_dup("."); + if (!dir) + return; + } else if (slash == dir) { + slash[1] = '\0'; /* keep the leading '/' */ + } else { + *slash = '\0'; + } + int dfd = open(dir, O_RDONLY | O_DIRECTORY | O_CLOEXEC); + free(dir); + if (dfd < 0) + return; + fsync(dfd); + close(dfd); +} + +/* Load the persistent dummy key for `store_path` from its `.dummykey` + * sidecar, creating it (exact mode 0600, 32 random bytes) if absent. A NULL + * store_path (empty store) yields a fresh ephemeral key. Reading an existing + * sidecar fails CLOSED on any validation error; only the CREATE path degrades + * to an ephemeral key (with a warning) when the filesystem cannot hold the + * sidecar (e.g. read-only mount), so a daemon still starts. + * + * Creation is ATOMIC: the key is written to a private same-directory temp file + * and hard-linked into place, so a concurrent starter (or reader) never observes + * a partial/zero sidecar that would fail the load closed. Returns false only + * when the CSPRNG itself fails (or a present-but-invalid sidecar is found). */ +static bool load_or_create_dummy_key(const char* store_path, uint8_t out[CREDENTIAL_KEY_LEN], + char* err, size_t err_size) { + if (!store_path) { + if (!credentials_random_bytes(out, CREDENTIAL_KEY_LEN)) { + set_error(err, err_size, "failed to generate the credential store dummy key"); + return false; + } + return true; + } + + size_t path_len = strlen(store_path); + size_t suffix_len = sizeof(CREDENTIAL_DUMMY_KEY_SUFFIX); /* includes the NUL */ + if (path_len > SIZE_MAX - suffix_len) { + set_error(err, err_size, "credential store path is too long to build a dummy key path"); + return false; + } + char* sidecar = malloc(path_len + suffix_len); + if (!sidecar) { + set_error(err, err_size, "out of memory building the dummy key path"); + return false; + } + int n = snprintf(sidecar, path_len + suffix_len, "%s%s", store_path, CREDENTIAL_DUMMY_KEY_SUFFIX); + if (n < 0 || (size_t)n >= path_len + suffix_len) { + set_error(err, err_size, "credential store path is too long to build a dummy key path"); + free(sidecar); + return false; + } + + /* Readers reject a planted symlink (O_NOFOLLOW) and never block on a planted + * FIFO (O_NONBLOCK; fstat rejects the non-regular file before any data read). + * Any open error other than ENOENT fails closed. */ + int fd = open(sidecar, O_RDONLY | O_NOFOLLOW | O_NONBLOCK | O_CLOEXEC); + if (fd >= 0) { + bool ok = read_dummy_key_fd(fd, sidecar, out, err, err_size); + close(fd); + free(sidecar); + return ok; + } + if (errno != ENOENT) { + /* The sidecar exists but cannot be opened for reading (EACCES, or ELOOP + * from a symlink): fail closed rather than substituting a different key. */ + set_error(err, err_size, "cannot open dummy key file '%s': %s", sidecar, strerror(errno)); + free(sidecar); + return false; + } + + /* Publish atomically: write a private same-directory temp file, fsync it, + * then hard-link it into place. A concurrent reader therefore only ever + * sees a complete 32-byte sidecar (or none), never a partial/zero file. + * + * The temp name carries both the pid and a fresh random suffix, so it is not + * predictable. If the name nevertheless already exists (a SIGKILL/crash + * leftover, pid reuse, or a planted file) the stale temp is removed and the + * O_EXCL create is retried once, so it can never silently defeat persistence + * for this pid. */ + uint8_t fresh[CREDENTIAL_KEY_LEN]; + if (!credentials_random_bytes(fresh, CREDENTIAL_KEY_LEN)) { + set_error(err, err_size, "failed to generate the credential store dummy key"); + free(sidecar); + return false; + } + + uint8_t name_rand[8]; + if (!credentials_random_bytes(name_rand, sizeof(name_rand))) { + set_error(err, err_size, "failed to generate the dummy key temp name"); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return false; + } + char name_hex[sizeof(name_rand) * 2 + 1]; + static const char hex_digits[] = "0123456789abcdef"; + for (size_t i = 0; i < sizeof(name_rand); i++) { + name_hex[2 * i] = hex_digits[name_rand[i] >> 4]; + name_hex[2 * i + 1] = hex_digits[name_rand[i] & 0x0f]; + } + name_hex[sizeof(name_hex) - 1] = '\0'; + + char tmp_suffix[64]; + int pn = snprintf(tmp_suffix, sizeof(tmp_suffix), ".tmp.%ld.%s", (long)getpid(), name_hex); + if (pn < 0 || (size_t)pn >= sizeof(tmp_suffix)) { + set_error(err, err_size, "failed to build the dummy key temp path"); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return false; + } + size_t sidecar_len = (size_t)n; + size_t tmp_len = sidecar_len + (size_t)pn; + char* tmp = malloc(tmp_len + 1); + if (!tmp) { + set_error(err, err_size, "out of memory building the dummy key temp path"); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return false; + } + snprintf(tmp, tmp_len + 1, "%s%s", sidecar, tmp_suffix); + + /* Bounded create: at most one unlink+retry on EEXIST. The retry keeps + * O_EXCL, so only a stale name is reclaimed and a live peer's temp is never + * truncated. */ + int create_errno = 0; + for (int attempt = 0; attempt < 2; attempt++) { + fd = open(tmp, O_WRONLY | O_CREAT | O_EXCL | O_CLOEXEC, 0600); + if (fd >= 0) + break; + create_errno = errno; + if (create_errno != EEXIST || attempt == 1) + break; + unlink(tmp); + } + /* umask can clear owner bits from the 0600 create mode while the reader + * requires an exact 0600, so force the mode on the fd before publishing; a + * failure here is treated like any other create failure (warning + ephemeral + * key) so the published sidecar is always exactly 0600. */ + if (fd >= 0 && fchmod(fd, 0600) != 0) { + create_errno = errno; + close(fd); + unlink(tmp); + fd = -1; + } + if (fd < 0) { + /* Creation failed (read-only filesystem, missing directory, fchmod, ...). + * Warn and fall back to an ephemeral key: unknown-user challenges stay + * deterministic within this daemon lifetime but will change on restart. */ + char* escaped = output_escape(tmp, log_get_8_bit_output()); + log_message(LOG_LEVEL_WARNING, + "cannot create dummy key file %s: %s; using a transient dummy key so unknown-user " + "challenges will change across restarts", + escaped ? escaped : tmp, strerror(create_errno)); + free(escaped); + memcpy(out, fresh, CREDENTIAL_KEY_LEN); + free(tmp); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return true; + } + + size_t written = 0; + bool write_ok = true; + int write_errno = 0; + while (written < CREDENTIAL_KEY_LEN) { + ssize_t w = write(fd, fresh + written, CREDENTIAL_KEY_LEN - written); + if (w < 0) { + if (errno == EINTR) + continue; + write_ok = false; + write_errno = errno; + break; + } + if (w == 0) { + /* A zero-length write is not a system error; errno is stale here, so + * report a clear short-write instead of a bogus strerror(errno). */ + write_ok = false; + write_errno = 0; + break; + } + written += (size_t)w; + } + if (write_ok && fsync(fd) != 0) { + write_ok = false; + write_errno = errno; + } + close(fd); + if (!write_ok) { + /* Do not leave a truncated temp file behind; fall back to an ephemeral key + * instead of failing closed on the next restart. */ + unlink(tmp); + char* escaped = output_escape(tmp, log_get_8_bit_output()); + const char* why = write_errno != 0 ? strerror(write_errno) : "short write"; + log_message(LOG_LEVEL_WARNING, + "cannot write dummy key file %s: %s; using a transient dummy key so unknown-user " + "challenges will change across restarts", + escaped ? escaped : tmp, why); + free(escaped); + memcpy(out, fresh, CREDENTIAL_KEY_LEN); + free(tmp); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return true; + } + + if (link(tmp, sidecar) != 0) { + int link_errno = errno; + if (link_errno == EEXIST) { + /* A concurrent starter published first; adopt its key. Read it back + * through the same hardened path (no symlink, no block, exact mode). */ + int rfd = open(sidecar, O_RDONLY | O_NOFOLLOW | O_NONBLOCK | O_CLOEXEC); + if (rfd < 0) { + set_error(err, err_size, "cannot open dummy key file '%s': %s", sidecar, strerror(errno)); + unlink(tmp); + free(tmp); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return false; + } + bool ok = read_dummy_key_fd(rfd, sidecar, out, err, err_size); + close(rfd); + unlink(tmp); + free(tmp); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return ok; + } + /* Linking failed for another reason (e.g. no hard-link support on this + * filesystem). Warn and fall back to an ephemeral key. */ + unlink(tmp); + char* escaped = output_escape(sidecar, log_get_8_bit_output()); + log_message(LOG_LEVEL_WARNING, + "cannot publish dummy key file %s: %s; using a transient dummy key so unknown-user " + "challenges will change across restarts", + escaped ? escaped : sidecar, strerror(link_errno)); + free(escaped); + memcpy(out, fresh, CREDENTIAL_KEY_LEN); + free(tmp); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return true; + } + + /* Published: make the new directory entry durable, then drop the private + * temp name (the sidecar keeps the inode alive). */ + fsync_containing_dir(sidecar); + unlink(tmp); + memcpy(out, fresh, CREDENTIAL_KEY_LEN); + free(tmp); + free(sidecar); + credentials_burn((char*)fresh, sizeof(fresh)); + return true; +} + CredentialStore* credentials_load(const char* password_file, const char* early_input_file, char* err, size_t err_size) { if (err && err_size) @@ -590,12 +905,15 @@ CredentialStore* credentials_load(const char* password_file, const char* early_i CredentialStore* store = load_store_file(password_file, err, err_size); if (!store) return NULL; - /* Generate the store-wide dummy key once for the final (possibly merged) - * store. It makes an unknown-user challenge deterministic, so fail the load - * if the CSPRNG is unavailable rather than degrading the anti-enumeration - * property. */ - if (!credentials_random_bytes(store->dummy_key, sizeof(store->dummy_key))) { - set_error(err, err_size, "failed to generate the credential store dummy key"); + /* Load (or create) the store-wide dummy key once for the final (possibly + * merged) store. It makes an unknown-user challenge deterministic AND + * stable across daemon restarts, so a restart cannot be used as a + * username-enumeration oracle. It is persisted in an exact-mode-0600 sidecar + * next to the credential store; a NULL store path (empty store) keeps it + * ephemeral. Fail the load if the CSPRNG is unavailable rather than + * degrading the anti-enumeration property. */ + const char* store_path = password_file ? password_file : early_input_file; + if (!load_or_create_dummy_key(store_path, store->dummy_key, err, err_size)) { credentials_free(store); return NULL; } diff --git a/src/shared/credentials.h b/src/shared/credentials.h index 9742ea9..116ff13 100644 --- a/src/shared/credentials.h +++ b/src/shared/credentials.h @@ -26,6 +26,17 @@ * hard-rejected with an actionable "legacy" error; there is no auto-upgrade. * Use `fastsync-server --hash-credentials` to generate new-format lines. * + * Alongside the store, credentials_load maintains an exact-mode-0600 + * `.dummykey` sidecar holding the store-wide random dummy key. It + * is auto-created on first load and MUST be preserved across restarts: it makes + * the dummy challenge for an unknown user stable for the life of the store, so + * a daemon restart cannot be used as a username-enumeration oracle. A sidecar + * that is not an exact-mode-0600 regular file of exactly 32 bytes fails the load + * (fail closed); if it cannot be created (e.g. a read-only mount or a restrictive + * umask the fchmod cannot repair) the daemon warns and uses a transient per-run + * key instead. NOTE: the sidecar requires EXACT 0600, whereas the store / + * password files only reject group/other bits (a deliberate difference). + * * Client --password-file format: the FIRST meaningful (non-comment, non-blank) * line is `user:password`, holding the literal password. The client keeps it * only for the duration of the handshake and wipes it at teardown; the file @@ -156,8 +167,9 @@ bool credentials_verify_response(const CredentialVerifier* v, const char* user, bool credentials_hash_store_line(const char* user, const char* password, uint32_t iters, char* out, size_t out_sz, char* err, size_t err_size); -/* Read `user:password` lines from `path` (the same owner-only check as the - * other secret files) and write one new-format store line per entry to `out`. +/* Read `user:password` lines from `path` (the same no-group/other-bits check as + * the other secret files) and write one new-format store line per entry to + * `out`. * Blank/comment lines are skipped; a malformed line fails the whole run. * Returns 0 on success, -1 on error (err filled). Used by * `--hash-credentials`. */ diff --git a/tests/test_credentials.c b/tests/test_credentials.c index 425a715..86d4dd6 100644 --- a/tests/test_credentials.c +++ b/tests/test_credentials.c @@ -3,6 +3,7 @@ #include "test_utils.h" #include "utils.h" #include +#include #include #include #include @@ -70,8 +71,43 @@ static char* make_tmp_file(const char* contents) { } static void rm_temp(const char* path) { - if (path) - unlink(path); + if (!path) + return; + unlink(path); + /* Every successfully loaded store auto-creates an exact-mode-0600 + * `.dummykey` sidecar; remove it too so tests leave no stray key. The + * atomic-publish temps carry a random suffix, so glob them all and remove any + * that a failing path may have left behind. */ + size_t n = strlen(path) + strlen(".dummykey") + 1; + char* sidecar = malloc(n); + if (sidecar) { + snprintf(sidecar, n, "%s.dummykey", path); + unlink(sidecar); + free(sidecar); + } + n = strlen(path) + strlen(".dummykey.tmp.*") + 1; + char* pattern = malloc(n); + if (pattern) { + snprintf(pattern, n, "%s.dummykey.tmp.*", path); + glob_t matches; + memset(&matches, 0, sizeof(matches)); + if (glob(pattern, 0, NULL, &matches) == 0) { + for (size_t i = 0; i < matches.gl_pathc; i++) + unlink(matches.gl_pathv[i]); + } + globfree(&matches); + free(pattern); + } +} + +/* `.dummykey` sidecar path (caller frees). */ +static char* dummy_sidecar_path(const char* store_path) { + size_t n = strlen(store_path) + strlen(".dummykey") + 1; + char* out = malloc(n); + if (!out) + return NULL; + snprintf(out, n, "%s.dummykey", store_path); + return out; } /* Build a valid new-format line for user/password at iters. */ @@ -745,6 +781,269 @@ static void test_credentials_rejects_group_or_other_accessible() { free(path); } +/* Loading a store auto-creates an owner-only `.dummykey` sidecar whose + * key is stable across reloads, so an unknown-user dummy salt is identical + * across two loads (the anti-restart enumeration property). */ +static void test_credentials_dummy_key_persisted() { + char line[CREDENTIAL_MAX_LINE]; + EXPECT_TRUE(make_store_line("alice", KAT_PASSWORD, CREDENTIAL_MIN_ITERS, line, sizeof(line))); + char contents[CREDENTIAL_MAX_LINE + 2]; + snprintf(contents, sizeof(contents), "%s\n", line); + char* path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + char* sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + + char err[512]; + CredentialStore* store = credentials_load(path, NULL, err, sizeof(err)); + EXPECT_NOT_NULL(store); + + struct stat st; + EXPECT_EQ_INT(stat(sidecar, &st), 0); + EXPECT_TRUE(S_ISREG(st.st_mode)); + EXPECT_TRUE((st.st_mode & (S_IRWXG | S_IRWXO)) == 0); + EXPECT_EQ_INT((int)(st.st_mode & 07777), 0600); + EXPECT_EQ_INT((int)st.st_size, CREDENTIAL_KEY_LEN); + + CredentialVerifier v1; + EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v1)); + EXPECT_FALSE(v1.found); + credentials_free(store); + + store = credentials_load(path, NULL, err, sizeof(err)); + EXPECT_NOT_NULL(store); + CredentialVerifier v2; + EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v2)); + EXPECT_FALSE(v2.found); + EXPECT_TRUE(memcmp(v1.salt, v2.salt, sizeof(v1.salt)) == 0); + credentials_free(store); + + rm_temp(path); + free(path); + free(sidecar); +} + +/* A restrictive umask must not leave the freshly published sidecar with owner + * bits cleared: creation forces exact 0600 with fchmod (the reader requires an + * exact 0600), so the daemon cannot lock itself out on the next restart. */ +static void test_credentials_dummy_key_exact_mode_under_umask() { + char line[CREDENTIAL_MAX_LINE]; + EXPECT_TRUE(make_store_line("alice", KAT_PASSWORD, CREDENTIAL_MIN_ITERS, line, sizeof(line))); + char contents[CREDENTIAL_MAX_LINE + 2]; + snprintf(contents, sizeof(contents), "%s\n", line); + char* path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + char* sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + + /* Clear every permission bit the O_CREAT mode would otherwise provide; only + * the explicit fchmod can restore the exact 0600 the reader demands. */ + mode_t old_umask = umask(0777); + char err[512]; + CredentialStore* store = credentials_load(path, NULL, err, sizeof(err)); + umask(old_umask); + EXPECT_NOT_NULL(store); + + struct stat st; + EXPECT_EQ_INT(stat(sidecar, &st), 0); + EXPECT_EQ_INT((int)(st.st_mode & 07777), 0600); + EXPECT_EQ_INT((int)st.st_size, CREDENTIAL_KEY_LEN); + credentials_free(store); + + rm_temp(path); + free(path); + free(sidecar); +} + +/* A sidecar that is group/other accessible, the wrong size, or not a regular + * file must fail the load closed. */ +static void test_credentials_dummy_key_rejects_bad_sidecar() { + char err[512]; + char line[CREDENTIAL_MAX_LINE]; + EXPECT_TRUE(make_store_line("alice", KAT_PASSWORD, CREDENTIAL_MIN_ITERS, line, sizeof(line))); + char contents[CREDENTIAL_MAX_LINE + 2]; + snprintf(contents, sizeof(contents), "%s\n", line); + + /* Group/other permission bits on the sidecar. */ + char* path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + char* sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + uint8_t key[CREDENTIAL_KEY_LEN]; + memset(key, 0x5a, sizeof(key)); + FILE* fp = fopen(sidecar, "wb"); + EXPECT_NOT_NULL(fp); + EXPECT_TRUE(fwrite(key, 1, sizeof(key), fp) == sizeof(key)); + fclose(fp); + EXPECT_EQ_INT(chmod(sidecar, 0640), 0); + EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err))); + EXPECT_TRUE(err[0] != '\0'); + rm_temp(path); + free(path); + free(sidecar); + + /* Wrong size (not exactly 32 bytes). */ + path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + fp = fopen(sidecar, "wb"); + EXPECT_NOT_NULL(fp); + EXPECT_TRUE(fwrite(key, 1, CREDENTIAL_SALT_LEN, fp) == CREDENTIAL_SALT_LEN); + fclose(fp); + EXPECT_EQ_INT(chmod(sidecar, 0600), 0); + EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err))); + EXPECT_TRUE(err[0] != '\0'); + rm_temp(path); + free(path); + free(sidecar); + + /* Non-regular file (a directory at the sidecar path). */ + path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + EXPECT_EQ_INT(mkdir(sidecar, 0700), 0); + EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err))); + EXPECT_TRUE(err[0] != '\0'); + rmdir(sidecar); + rm_temp(path); + free(path); + free(sidecar); + + /* Exact-mode rule: 0400 has no group/other bits but is not 0600, so it is + * rejected now (the mode must be exactly owner read+write). */ + path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + fp = fopen(sidecar, "wb"); + EXPECT_NOT_NULL(fp); + EXPECT_TRUE(fwrite(key, 1, sizeof(key), fp) == sizeof(key)); + fclose(fp); + EXPECT_EQ_INT(chmod(sidecar, 0400), 0); + EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err))); + EXPECT_TRUE(err[0] != '\0'); + rm_temp(path); + free(path); + free(sidecar); +} + +/* A pre-existing valid sidecar is adopted verbatim (no regeneration): the + * unknown-user dummy salt must equal HMAC-SHA256(known key, username), and a + * reload must yield the same salt. */ +static void test_credentials_dummy_key_existing_sidecar_adopted() { + char line[CREDENTIAL_MAX_LINE]; + EXPECT_TRUE(make_store_line("alice", KAT_PASSWORD, CREDENTIAL_MIN_ITERS, line, sizeof(line))); + char contents[CREDENTIAL_MAX_LINE + 2]; + snprintf(contents, sizeof(contents), "%s\n", line); + char* path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + char* sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + + /* Pre-create a valid owner-only sidecar with a known key. */ + uint8_t key[CREDENTIAL_KEY_LEN]; + memset(key, 0x5a, sizeof(key)); + FILE* fp = fopen(sidecar, "wb"); + EXPECT_NOT_NULL(fp); + EXPECT_TRUE(fwrite(key, 1, sizeof(key), fp) == sizeof(key)); + fclose(fp); + EXPECT_EQ_INT(chmod(sidecar, 0600), 0); + + char err[512]; + CredentialStore* store = credentials_load(path, NULL, err, sizeof(err)); + EXPECT_NOT_NULL(store); + CredentialVerifier v1; + EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v1)); + EXPECT_FALSE(v1.found); + /* HMAC-SHA256(0x5a * 32, "unknown-user")[:16], computed independently. */ + uint8_t expect[CREDENTIAL_SALT_LEN]; + unhex("4b0d2e6b73025cc2fcb41d0a710ff469", expect, sizeof(expect)); + EXPECT_TRUE(memcmp(v1.salt, expect, sizeof(expect)) == 0); + credentials_free(store); + + /* The adopted sidecar still holds exactly the pre-created key (not a fresh + * random one). */ + uint8_t readback[CREDENTIAL_KEY_LEN]; + fp = fopen(sidecar, "rb"); + EXPECT_NOT_NULL(fp); + EXPECT_TRUE(fread(readback, 1, sizeof(readback), fp) == sizeof(readback)); + fclose(fp); + EXPECT_TRUE(memcmp(readback, key, sizeof(key)) == 0); + + /* Persisted across a reload. */ + CredentialVerifier v2; + store = credentials_load(path, NULL, err, sizeof(err)); + EXPECT_NOT_NULL(store); + EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v2)); + EXPECT_TRUE(memcmp(v1.salt, v2.salt, sizeof(v1.salt)) == 0); + credentials_free(store); + + rm_temp(path); + free(path); + free(sidecar); +} + +/* A symlink planted at the sidecar path must fail the load closed (O_NOFOLLOW), + * even when it resolves to a valid owner-only key file. */ +static void test_credentials_dummy_key_symlink_rejected() { + char line[CREDENTIAL_MAX_LINE]; + EXPECT_TRUE(make_store_line("alice", KAT_PASSWORD, CREDENTIAL_MIN_ITERS, line, sizeof(line))); + char contents[CREDENTIAL_MAX_LINE + 2]; + snprintf(contents, sizeof(contents), "%s\n", line); + char* path = make_tmp_file(contents); + EXPECT_NOT_NULL(path); + char* sidecar = dummy_sidecar_path(path); + EXPECT_NOT_NULL(sidecar); + + char target[256]; + snprintf(target, sizeof(target), "/tmp/fs_cred_key_%d_%d", (int)getpid(), g_file_counter++); + uint8_t key[CREDENTIAL_KEY_LEN]; + memset(key, 0x5a, sizeof(key)); + FILE* fp = fopen(target, "wb"); + EXPECT_NOT_NULL(fp); + EXPECT_TRUE(fwrite(key, 1, sizeof(key), fp) == sizeof(key)); + fclose(fp); + EXPECT_EQ_INT(chmod(target, 0600), 0); + EXPECT_EQ_INT(symlink(target, sidecar), 0); + + char err[512]; + EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err))); + EXPECT_TRUE(err[0] != '\0'); + + unlink(sidecar); /* remove the symlink itself, not its target */ + unlink(target); + rm_temp(path); + free(path); + free(sidecar); +} + +/* A NULL store path has nowhere to persist a key, so each load gets a fresh + * ephemeral key (and creates no sidecar). */ +static void test_credentials_dummy_key_null_store_ephemeral() { + char err[512]; + CredentialStore* store = credentials_load(NULL, NULL, err, sizeof(err)); + EXPECT_NOT_NULL(store); + CredentialVerifier v1; + CredentialVerifier v1b; + EXPECT_TRUE(credentials_get_verifier(store, "nobody", NULL, 0, &v1)); + EXPECT_FALSE(v1.found); + /* Within one store the dummy challenge is still deterministic. */ + EXPECT_TRUE(credentials_get_verifier(store, "nobody", NULL, 0, &v1b)); + EXPECT_TRUE(memcmp(v1.salt, v1b.salt, sizeof(v1.salt)) == 0); + credentials_free(store); + + store = credentials_load(NULL, NULL, err, sizeof(err)); + EXPECT_NOT_NULL(store); + CredentialVerifier v2; + EXPECT_TRUE(credentials_get_verifier(store, "nobody", NULL, 0, &v2)); + /* No persistence path, so the second load's random key differs (and with it + * the dummy salt). */ + EXPECT_TRUE(memcmp(v1.salt, v2.salt, sizeof(v1.salt)) != 0); + credentials_free(store); +} + static void test_credentials_burn() { char secret[32]; memcpy(secret, "supersecretvalue", 17); @@ -777,5 +1076,11 @@ void test_credentials(void) { test_credentials_read_secret_file_bad(); test_credentials_hash_file(); test_credentials_rejects_group_or_other_accessible(); + test_credentials_dummy_key_persisted(); + test_credentials_dummy_key_exact_mode_under_umask(); + test_credentials_dummy_key_rejects_bad_sidecar(); + test_credentials_dummy_key_existing_sidecar_adopted(); + test_credentials_dummy_key_symlink_rejected(); + test_credentials_dummy_key_null_store_ephemeral(); test_credentials_burn(); }