[PATCH] keys: Fix UAF in key_put()

[David Howells]

From: Hillf Danton <hdanton at sina.com>
    
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 on an expedited basis[*] by taking a ref on the key->user struct
and caching the key length before dropping the refcount so that we can
reduce the quota afterwards if we dropped the last ref.

[*] This is going to hurt key_put() performance, so a better way is
probably necessary, such as sticking the dead key onto a queue for the
garbage collector to pick up rather than having it scan the serial number
registry.

Fixes: 9578e327b2b4 ("keys: update key quotas in key_put()")
Reported-by: syzbot+6105ffc1ded71d194d6d at syzkaller.appspotmail.com
Tested-by: syzbot+6105ffc1ded71d194d6d at syzkaller.appspotmail.com
Suggested-by: Hillf Danton <hdanton at sina.com>
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Jarkko Sakkinen <jarkko at kernel.org>
cc: Kees Cook <kees at kernel.org>
cc: keyrings at vger.kernel.org
cc: stable at vger.kernel.org # v6.10+
---
 security/keys/key.c |   19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/security/keys/key.c b/security/keys/key.c
index 3d7d185019d3..1e6028492355 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -645,21 +645,30 @@ EXPORT_SYMBOL(key_reject_and_link);
  */
 void key_put(struct key *key)
 {
+	int quota_flag;
+	unsigned short len;
+	struct key_user *user;
+
 	if (key) {
 		key_check(key);
 
+		quota_flag = test_bit(KEY_FLAG_IN_QUOTA, &key->flags);
+		len = key->quotalen;
+		user = key->user;
+		refcount_inc(&user->usage);
 		if (refcount_dec_and_test(&key->usage)) {
 			unsigned long flags;
 
 			/* deal with the user's key tracking and quota */
-			if (test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
-				spin_lock_irqsave(&key->user->lock, flags);
-				key->user->qnkeys--;
-				key->user->qnbytes -= key->quotalen;
-				spin_unlock_irqrestore(&key->user->lock, flags);
+			if (quota_flag) {
+				spin_lock_irqsave(&user->lock, flags);
+				user->qnkeys--;
+				user->qnbytes -= len;
+				spin_unlock_irqrestore(&user->lock, flags);
 			}
 			schedule_work(&key_gc_work);
 		}
+		key_user_put(user);
 	}
 }
 EXPORT_SYMBOL(key_put);