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

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> 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();

It looks like we can drop the rcu_read_lock()/unlock() section then if we
take a reference on the psock? Before it was there to ensure we didn't
lose the psock from some other context, but with a reference held this
can not happen.

> -	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
>