Re: [syzbot] [keyrings?] [lsm?] KASAN: slab-use-after-free Read in key_put

[David Howells]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

commit 91286ccf56bb5030c0c84b11684f06142f50e681
Author: David Howells <dhowells@xxxxxxxxxx>
Date:   Tue Mar 18 09:54:54 2025 +0000

    keys: Fix UAF in key_put()
    
    Once a key's reference count has been reduced to 0, the garbage collector
    thread may destroy it at any time and so key_put() is not allowed to touch
    the key after that point.  The most key_put() is normally allowed to do is
    to touch key_gc_work as that's a static global variable.
    
    However, in an effort to speed up the reclamation of quota, this is now
    done in key_put() once the key's usage is reduced to 0 - but now the code
    is looking at the key after the deadline, which is forbidden.
    
    Fix this by using a flag to indicate that a key can be gc'd now rather than
    looking at the key's refcount in the garbage collector.
    
    Fixes: 9578e327b2b4 ("keys: update key quotas in key_put()")
    Reported-by: syzbot+6105ffc1ded71d194d6d@xxxxxxxxxxxxxxxxxxxxxxxxx
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
    cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
    cc: Oleg Nesterov <oleg@xxxxxxxxxx>
    cc: Kees Cook <kees@xxxxxxxxxx>
    cc: Hillf Danton <hdanton@xxxxxxxx>,
    cc: keyrings@xxxxxxxxxxxxxxx
    Cc: stable@xxxxxxxxxxxxxxx # v6.10+

diff --git a/include/linux/key.h b/include/linux/key.h
index 074dca3222b9..ba05de8579ec 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -236,6 +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 */
 
 	/* 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 7d687b0962b1..f27223ea4578 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -218,8 +218,10 @@ static void key_garbage_collector(struct work_struct *work)
 		key = rb_entry(cursor, struct key, serial_node);
 		cursor = rb_next(cursor);
 
-		if (refcount_read(&key->usage) == 0)
+		if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) {
+			smp_mb(); /* Clobber key->user after FINAL_PUT seen. */
 			goto found_unreferenced_key;
+		}
 
 		if (unlikely(gc_state & KEY_GC_REAPING_DEAD_1)) {
 			if (key->type == key_gc_dead_keytype) {
diff --git a/security/keys/key.c b/security/keys/key.c
index 3d7d185019d3..7198cd2ac3a3 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -658,6 +658,8 @@ 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);
 			schedule_work(&key_gc_work);
 		}
 	}