[Patch bpf-next] sock_map: fix a potential use-after-free in sock_map_close()

[Cong Wang <xiyou.wangcong@gmail.com>]

From: Cong Wang <cong.wang@bytedance.com>

The last refcnt of the psock can be gone right after
sock_map_remove_links(), so sk_psock_stop() could trigger a UAF.
The reason why I placed sk_psock_stop() there is to avoid RCU read
critical section, and more importantly, some callee of
sock_map_remove_links() is supposed to be called with RCU read lock,
we can not simply get rid of RCU read lock here. Therefore, the only
choice we have is to grab an additional refcnt with sk_psock_get()
and put it back after sk_psock_stop().

Reported-by: syzbot+7b6548ae483d6f4c64ae@syzkaller.appspotmail.com
Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jakub Sitnicki <jakub@cloudflare.com>
Cc: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
---
 net/core/sock_map.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index f473c51cbc4b..6f1b82b8ad49 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -1521,7 +1521,7 @@ void sock_map_close(struct sock *sk, long timeout)
 
 	lock_sock(sk);
 	rcu_read_lock();
-	psock = sk_psock(sk);
+	psock = sk_psock_get(sk);
 	if (unlikely(!psock)) {
 		rcu_read_unlock();
 		release_sock(sk);
@@ -1532,6 +1532,7 @@ void sock_map_close(struct sock *sk, long timeout)
 	sock_map_remove_links(sk, psock);
 	rcu_read_unlock();
 	sk_psock_stop(psock, true);
+	sk_psock_put(sk, psock);
 	release_sock(sk);
 	saved_close(sk, timeout);
 }
-- 
2.25.1