[PATCH v2] keys: finalize persistent keyring timeout after link attempt

[Karl Mehltretter <kmehltretter@gmail.com>]

When no keyring exists for the requested UID, KEYCTL_GET_PERSISTENT
creates and registers one before linking it to the requested destination.
The configured timeout is set only after the destination link succeeds.

A destination restricted with KEYCTL_RESTRICT_KEYRING makes that link fail
with -EPERM. With persistent_keyring_expiry set to 60 seconds, /proc/keys
still reports the registered keyring's expiry as "perm".
The failed call therefore leaves a quota-exempt keyring in the namespace's
hidden register, where it may remain until namespace teardown.

Have key_create_persistent() report whether it allocated a new keyring.
Another caller may create one between the initial read-locked lookup and
the retry under the write lock. Set the timeout after permission checking
and linking. Do this on success, or on failure if the call allocated the
keyring.

This leaves a new keyring collectible after failure without starting its
timeout while linking is still in progress.

Fixes: f36f8c75ae2e ("KEYS: Add per-user_namespace registers for persistent per-UID kerberos caches")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Changes in v2:
- Rewrite the commit message around the restricted-destination reproducer
  and trim the explanation (Jarkko).
- Track whether the persistent keyring was newly allocated and finalize its
  timeout after the permission and link checks. This preserves an existing
  keyring's timeout on failure and avoids expiry during linking.

Tested with QEMU 10.2.1 TCG on i386 and x86_64. With
persistent_keyring_expiry set to 60 seconds, KEYCTL_GET_PERSISTENT returned
-EPERM and /proc/keys reported "perm" before the fix and "1m" after it on
both architectures.

Also tested on x86_64 with persistent_keyring_expiry set to 1 second and
gc_delay set to 0. Successful calls reused the same serial, a newly created
keyring from a failed call expired, and a failed call using an existing
keyring left its timeout unchanged. A test-only two-second delay before
timeout finalization did not allow the keyring to be collected before link.

 security/keys/persistent.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/security/keys/persistent.c b/security/keys/persistent.c
index 97af230aa4b22..f356df8f3c973 100644
--- a/security/keys/persistent.c
+++ b/security/keys/persistent.c
@@ -38,7 +38,8 @@ static int key_create_persistent_register(struct user_namespace *ns)
  * Called with the namespace's sem locked for writing.
  */
 static key_ref_t key_create_persistent(struct user_namespace *ns, kuid_t uid,
-				       struct keyring_index_key *index_key)
+				       struct keyring_index_key *index_key,
+				       bool *created)
 {
 	struct key *persistent;
 	key_ref_t reg_ref, persistent_ref;
@@ -63,6 +64,8 @@ static key_ref_t key_create_persistent(struct user_namespace *ns, kuid_t uid,
 	if (IS_ERR(persistent))
 		return ERR_CAST(persistent);
 
+	*created = true;
+
 	return make_key_ref(persistent, true);
 }
 
@@ -78,6 +81,7 @@ static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
 	key_ref_t reg_ref, persistent_ref;
 	char buf[32];
 	long ret;
+	bool created = false;
 
 	/* Look in the register if it exists */
 	memset(&index_key, 0, sizeof(index_key));
@@ -100,7 +104,7 @@ static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
 	 * also need to create the register.
 	 */
 	down_write(&ns->keyring_sem);
-	persistent_ref = key_create_persistent(ns, uid, &index_key);
+	persistent_ref = key_create_persistent(ns, uid, &index_key, &created);
 	up_write(&ns->keyring_sem);
 	if (!IS_ERR(persistent_ref))
 		goto found;
@@ -108,15 +112,16 @@ static long key_get_persistent(struct user_namespace *ns, kuid_t uid,
 	return PTR_ERR(persistent_ref);
 
 found:
+	persistent = key_ref_to_ptr(persistent_ref);
 	ret = key_task_permission(persistent_ref, current_cred(), KEY_NEED_LINK);
-	if (ret == 0) {
-		persistent = key_ref_to_ptr(persistent_ref);
+	if (ret == 0)
 		ret = key_link(key_ref_to_ptr(dest_ref), persistent);
-		if (ret == 0) {
-			key_set_timeout(persistent, persistent_keyring_expiry);
-			ret = persistent->serial;
-		}
-	}
+
+	if (ret == 0 || created)
+		key_set_timeout(persistent, persistent_keyring_expiry);
+
+	if (ret == 0)
+		ret = persistent->serial;
 
 	key_ref_put(persistent_ref);
 	return ret;

base-commit: 08dbfad3f5040f5bdb6c529da20d6d4e81fefd72
-- 
2.53.0