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keys: Fix UAF in key_put()

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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: 9578e327b2 ("keys: update key quotas in key_put()")
Reported-by: syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/673b6aec.050a0220.87769.004a.GAE@google.com/
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: syzbot+6105ffc1ded71d194d6d@syzkaller.appspotmail.com
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
This commit is contained in:
David Howells 2025-03-19 15:57:46 +00:00 • committed by Jarkko Sakkinen
parent 88d324e69e
commit 75845c6c1a
3 changed files with 6 additions and 1 deletions
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1
include/linux/key.h
View file

@ -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

4
security/keys/gc.c
View file

@ -218,8 +218,10 @@ continue_scanning:
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) {

2
security/keys/key.c
View file

@ -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);
}
}