KEYS: Fix add_key() race with keyring restriction

__key_create_or_update() snapshots keyring->restrict_link before taking
the destination keyring's semaphore.  keyring_restrict() installs a
restriction while holding that semaphore.

This allows a writer to observe no restriction, wait for the keyring
owner to install a reject-all restriction and return successfully, and
then link a key using the stale NULL snapshot.  The writer only needs
write permission on the destination keyring.

Move the restrict_link read after __key_link_lock() and
__key_link_begin().  The read and the subsequent restriction check are
then serialized with restriction installation by keyring->sem.

The race was reproduced on v7.2.8 in 19 executions where restriction
installation returned before the link completed.  All 19 linked the key
despite the reject-all restriction.  With this change, 312 executions
reached the same ordering and every add_key() call failed with -EPERM.

The issue was found by manual concurrency analysis assisted by AI-based
analysis and independently verified with a QEMU reproducer and kernel
instrumentation.

Fixes: 5ac7eace2d ("KEYS: Add a facility to restrict new links into a keyring")
Cc: [email protected]
Signed-off-by: 성병찬 <[email protected]>
Reviewed-by: Jarkko Sakkinen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jarkko Sakkinen <[email protected]>

security/keys/key.c

@@ -840,9 +840,6 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
 
 	key_check(keyring);
 
-	if (!(flags & KEY_ALLOC_BYPASS_RESTRICTION))
-		restrict_link = keyring->restrict_link;
-
 	key_ref = ERR_PTR(-ENOTDIR);
 	if (keyring->type != &key_type_keyring)
 		goto error_put_type;
@@ -880,6 +877,9 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
 		goto error_link_end;
 	}
 
+	if (!(flags & KEY_ALLOC_BYPASS_RESTRICTION))
+		restrict_link = keyring->restrict_link;
+
 	if (restrict_link && restrict_link->check) {
 		ret = restrict_link->check(keyring, index_key.type,
 					   &prep.payload, restrict_link->key);