[v3 PATCH] KEYS: Invert FINAL_PUT bit

Herbert Xu
2025-05-09T09:45:29Z

On Fri, May 09, 2025 at 05:34:09PM +0800, kernel test robot wrote:
>
> our bot applied this patch directly upon v6.15-rc5. could you let us know if
> this is a correct appliment?

Yes it is correct.

The patch was buggy, the test_bit_acquire should've been inverted too:

---8<---
Invert the FINAL_PUT bit so that test_bit_acquire and clear_bit_unlock
can be used instead of smp_mb.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/include/linux/key.h b/include/linux/key.h
index ba05de8579ec..81b8f05c6898 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -236,7 +236,7 @@ struct key {
#define KEY_FLAG_ROOT_CAN_INVAL 7 /* set if key can be invalidated by root without permission */
#define KEY_FLAG_KEEP 8 /* set if key should not be removed */
#define KEY_FLAG_UID_KEYRING 9 /* set if key is a user or user session keyring */
-#define KEY_FLAG_FINAL_PUT 10 /* set if final put has happened on key */
+#define KEY_FLAG_USER_ALIVE 10 /* set if final put has not happened on key yet */

/* the key type and key description string
* - the desc is used to match a key against search criteria
diff --git a/security/keys/gc.c b/security/keys/gc.c
index f27223ea4578..748e83818a76 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -218,8 +218,8 @@ static void key_garbage_collector(struct work_struct *work)
key = rb_entry(cursor, struct key, serial_node);
cursor = rb_next(cursor);

- if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) {
- smp_mb(); /* Clobber key->user after FINAL_PUT seen. */
+ if (!test_bit_acquire(KEY_FLAG_USER_ALIVE, &key->flags)) {
+ /* Clobber key->user after final put seen. */
goto found_unreferenced_key;
}

diff --git a/security/keys/key.c b/security/keys/key.c
index 7198cd2ac3a3..3bbdde778631 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -298,6 +298,7 @@ struct key *key_alloc(struct key_type *type, const char *desc,
key->restrict_link = restrict_link;
key->last_used_at = ktime_get_real_seconds();

+ key->flags |= 1 << KEY_FLAG_USER_ALIVE;
if (!(flags & KEY_ALLOC_NOT_IN_QUOTA))
key->flags |= 1 << KEY_FLAG_IN_QUOTA;
if (flags & KEY_ALLOC_BUILT_IN)
@@ -658,8 +659,8 @@ void key_put(struct key *key)
key->user->qnbytes -= key->quotalen;
spin_unlock_irqrestore(&key->user->lock, flags);
}
- smp_mb(); /* key->user before FINAL_PUT set. */
- set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
+ /* Mark key as safe for GC after key->user done. */
+ clear_bit_unlock(KEY_FLAG_USER_ALIVE, &key->flags);
schedule_work(&key_gc_work);
}
}
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt