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fix(server): reject --allow-super with --stdio, fix module host-list append

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Re-review findings on the C3/C4 hardening branch:

- --stdio is the SSH transport whose remote argv is composed by the client
  (including via --remote-option), so accepting --allow-super there let a
  client defeat the C3 secure default for a root receiver.  Reject it at CLI
  parse time (standalone TCP only) and force the process-global flag off for
  --stdio as defense in depth.  Correct the help text and README/RSYNC_COMPAT:
  the --stdio argv is client-composed, super stays off, and a forced command is
  needed if the default must hold.
- daemon_conf: the per-module 'hosts allow'/'hosts deny' call sites passed
  module_name and replace in the wrong order, so multiple lines replaced
  instead of appended and the empty-value error omitted the module name.  Pass
  (module->name, false) like the global keys; add a unit test for two
  per-module allow/deny lines appending.
- tls: read the client CN via ASN1_STRING_to_UTF8 so an exactly-required-length
  name is accepted and only actual over-length CNs are rejected.
This commit is contained in:
TapTap
2026-09-14 17:16:01 +02:00
parent 9da5a0a9ed
commit 825ba69753
8 changed files with 118 additions and 50 deletions
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src/server/server.c
+46 -29
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@@ -37,11 +37,14 @@ 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. */
/* --allow-super: locally-launched standalone TCP 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. It is REJECTED for --stdio (the SSH remote
* argv is composed by the client, so it must never be able to opt a root
* receiver back into super mode); the --stdio path always keeps the secure
* default. */
static bool server_allow_super;
static const char* required_client_cn;
/* --iconv CONVERT_SPEC the server was itself started with (borrowed argv
@@ -193,17 +196,25 @@ static bool tls_client_identity_allowed(SSL* ssl) {
X509* certificate = SSL_get1_peer_certificate(ssl);
if (!certificate)
return false;
char common_name[256];
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);
/* 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);
bool allowed = false;
X509_NAME* subject = X509_get_subject_name(certificate);
int index = subject ? X509_NAME_get_index_by_NID(subject, NID_commonName, -1) : -1;
if (index >= 0) {
X509_NAME_ENTRY* entry = X509_NAME_get_entry(subject, index);
ASN1_STRING* data = entry ? X509_NAME_ENTRY_get_data(entry) : NULL;
/* Convert the CN to UTF-8 to get its FULL byte length: unlike
* X509_NAME_get_text_by_NID (which truncates an over-long CN to the buffer
* and reports the truncated length), ASN1_STRING_to_UTF8 never truncates, so
* an exactly-required-length CN is accepted while an over-long one cannot be
* prefix-matched by a shorter required name. */
unsigned char* utf8 = NULL;
int cn_length = data ? ASN1_STRING_to_UTF8(&utf8, data) : -1;
if (cn_length >= 0 && (size_t)cn_length == required_length)
allowed = credentials_secure_equal((const char*)utf8, required_client_cn, required_length);
if (utf8)
OPENSSL_free(utf8);
}
X509_free(certificate);
return allowed;
}
@@ -602,15 +613,17 @@ 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. */
/* C3: a privileged (root) STANDALONE 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; the flag is rejected for
* --stdio, whose client-composed argv must never defeat this default (an
* operator exposing `fastsync-server --stdio` over SSH needs a forced command
* to keep the 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)
@@ -1041,11 +1054,13 @@ 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(" --allow-super Standalone TCP listener only: keep super-user\n");
printf(" activities enabled for a root receiver. Without it a\n");
printf(" root 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(" requests are refused/skipped. Never honored with\n");
printf(" --stdio (the SSH remote argv is client-composed, so\n");
printf(" super stays off there); 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");
@@ -1170,7 +1185,9 @@ 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;
/* --stdio rejects --allow-super at parse time; force it off here as well so
* this process-global policy cannot be re-enabled by a future caller. */
server_allow_super = opts.allow_super && !opts.stdio_mode;
server_iconv_spec = opts.iconv_spec;
signal(SIGINT, cleanup);
signal(SIGTERM, cleanup);