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fix(server): gate --force by --allow-delete and secure root super default

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C2: --force is deletion authority (an incoming regular file may remove a
non-empty destination directory tree, and --delete-missing-args may
remove a non-empty directory mirror), but it was not masked by the
operator --allow-delete policy.  The handler now clears
config->force_delete unless --allow-delete was given, exactly like
--delete and --delete-missing-args.

C3: a standalone TCP / --stdio server running as root defaulted to
SUPER_MODE_AUTO, so an untrusted client --devices/--write-devices/
--super could make it create device nodes, write raw devices, or apply
client-chosen ownership.  A privileged standalone receiver now forces
SUPER_MODE_OFF unless the operator opts in with the new server-only
--allow-super flag.  Non-root receivers are unchanged, and the daemon
path keeps its per-module `client owner = yes` gate.  --allow-super is
rejected with --no-super or --daemon.

C6: tls_client_identity_allowed now rejects a CN whose reported length
reached the buffer bound, so a truncated over-long CN cannot be matched
by a required --client-cn prefix.

Tests: an integration regression proving --force cannot replace a
destination directory without --allow-delete; standalone-default tests
for --copy-as refusal and (root-only) skipped device creation; a CLI
unit test for the new flag.  The integration shared_server fixture opts
in with --allow-super so the existing root-only ownership/device/copy-as
tests continue to exercise the opted-in configuration.  README and
RSYNC_COMPAT document the flag and the force/delete gating.
This commit is contained in:
TapTap
2026-09-14 16:09:44 +02:00
parent 80c1ff321c
commit 9da5a0a9ed
8 changed files with 180 additions and 12 deletions
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src/server/server.c
+39 -2
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@@ -37,6 +37,12 @@ static bool allow_unauthenticated;
* root), so no super-user activity is attempted and any client --copy-as is
* refused. Set once in main before the accept loop / stdio handler. */
static bool server_no_super;
/* --allow-super: standalone/--stdio opt-in that preserves the historical
* permissive super mode for a root receiver. When false, a privileged
* standalone receiver forces SUPER_MODE_OFF for every connection (C3), so a
* client cannot make it create device nodes / write raw devices / apply
* client-chosen ownership. */
static bool server_allow_super;
static const char* required_client_cn;
/* --iconv CONVERT_SPEC the server was itself started with (borrowed argv
* pointer). Its LOCAL half may override the local charset the client assumed;
@@ -191,8 +197,12 @@ static bool tls_client_identity_allowed(SSL* ssl) {
int length = X509_NAME_get_text_by_NID(X509_get_subject_name(certificate), NID_commonName,
common_name, sizeof(common_name));
size_t required_length = strlen(required_client_cn);
bool allowed = length >= 0 && (size_t)length == required_length &&
required_length < sizeof(common_name) &&
/* X509_NAME_get_text_by_NID truncates an over-long CN to the buffer size; a
* returned length at the buffer bound means the CN was silently shortened, so
* a required-name prefix could be matched by a longer CN with extra suffix.
* Reject any result that reached the bound. */
bool allowed = length >= 0 && (size_t)length < sizeof(common_name) - 1 &&
(size_t)length == required_length && required_length < sizeof(common_name) &&
credentials_secure_equal(common_name, required_client_cn, required_length);
X509_free(certificate);
return allowed;
@@ -592,6 +602,20 @@ static const char* server_module_gate(const Config* config, void* context) {
if (gate_ctx)
gate_ctx->super_mode_override = SUPER_MODE_OFF;
}
/* C3: a privileged (root) STANDALONE/--stdio receiver defaults to
* SUPER_MODE_OFF. Without this a client --devices/--write-devices/--super
* would let a root server create arbitrary device nodes and write raw devices,
* and client-chosen ownership (--numeric-ids/--chown/--usermap/--groupmap)
* would be applied, with no operator opt-in. The operator must pass
* --allow-super to restore the historical permissive behavior; an
* unprivileged receiver is unaffected (the kernel refuses the confined
* attempts) and the daemon path keeps its per-module `client owner = yes`
* gate. */
if (g_daemon_conf == NULL && geteuid() == 0 && !server_allow_super) {
effective.super_mode = SUPER_MODE_OFF;
if (gate_ctx)
gate_ctx->super_mode_override = SUPER_MODE_OFF;
}
/* --copy-as (P7 Wave E, protocol 2.18.0): FastSync's safe subset forces the
ownership of every written entry to the requested ids, which needs a
privileged (root) receiver. An unprivileged receiver REFUSES the whole
@@ -751,6 +775,13 @@ void handler(int file_descriptor) {
goto done;
}
config->use_delete = config->use_delete && allow_delete;
/* --force (receiver-side) is deletion authority too: it lets an incoming
* regular file recursively remove a non-empty destination directory tree, and
* lets --delete-missing-args remove a non-empty directory mirror. Without
* the operator's --allow-delete it must be inert, exactly like --delete and
* --delete-missing-args, so a client cannot use --force to bypass the delete
* policy. */
config->force_delete = config->force_delete && allow_delete;
/* --iconv (protocol 2.16.0): install the receiver-side wire->local conversion
now that the client's full CONVERT_SPEC has been received and validated,
before any received file name is decoded. The server's own --iconv (if
@@ -1010,6 +1041,11 @@ static void print_server_usage(void) {
printf(" --no-super Operator veto: never attempt super-user activities\n");
printf(" (ownership, device nodes) even as root, and refuse\n");
printf(" any client --copy-as/--super request\n");
printf(" --allow-super Standalone/--stdio only: keep super-user activities\n");
printf(" enabled for a root receiver. Without it a root\n");
printf(" standalone server forces SUPER_MODE_OFF, so client\n");
printf(" --devices/--write-devices/--super and ownership\n");
printf(" requests are refused/skipped. No effect when not root\n");
printf(" --iconv=LOCAL[,REMOTE] Declare this server's LOCAL charset for file-name\n");
printf(" conversion: received names are translated to this\n");
printf(" charset (the wire charset still comes from the\n");
@@ -1134,6 +1170,7 @@ int main(int argc, char* argv[]) {
trust_sender = opts.trust_sender;
allow_unauthenticated = opts.allow_unauthenticated;
server_no_super = opts.no_super;
server_allow_super = opts.allow_super;
server_iconv_spec = opts.iconv_spec;
signal(SIGINT, cleanup);
signal(SIGTERM, cleanup);