[PATCH v1 net-next 2/2] af_unix: Don't hold client's sk_peer_lock in copy_peercred().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Kuniyuki Iwashima <kuniyu@amazon.com>
Date: Sun, 28 Jan 2024 02:37:32 -0800
> When (AF_UNIX, SOCK_STREAM) socket connect()s to a listening socket,
> the listener's sk_peer_pid/sk_peer_cred are copied to the client in
> copy_peercred().
> 
> Then, two sk_peer_locks are held there; one is client's and another
> is listener's.  However, we need not hold the client's lock.
> 
> The only place where the client's sk_peer_pid/sk_peer_cred are set
> is copy_peercred(), and once they are set, they never change.
> 
> Let's not hold client's sk_peer_lock in copy_peercred().
> 
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> ---
>  net/unix/af_unix.c | 20 ++------------------
>  1 file changed, 2 insertions(+), 18 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index f07374d31f7c..c5c292393be8 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -706,26 +706,10 @@ static void update_peercred(struct sock *sk)
>  
>  static void copy_peercred(struct sock *sk, struct sock *peersk)
>  {
> -	const struct cred *old_cred;
> -	struct pid *old_pid;
> -
> -	if (sk < peersk) {
> -		spin_lock(&sk->sk_peer_lock);
> -		spin_lock_nested(&peersk->sk_peer_lock, SINGLE_DEPTH_NESTING);
> -	} else {
> -		spin_lock(&peersk->sk_peer_lock);
> -		spin_lock_nested(&sk->sk_peer_lock, SINGLE_DEPTH_NESTING);

Somehow I forgot about that the client's sk_peer_pid could be
fetched at the same time by SO_PEERCRED/SO_PEERPIDFD, so the
lock is needed.


> -	}
> -	old_pid = sk->sk_peer_pid;
> -	old_cred = sk->sk_peer_cred;

but old_pid/old_cred are always NULL, so this can be removed.

Also, the locking order can be fixed as

  peer (TCP_LISTEN) -> sk (TCP_ESTABLISHED)

and we can write a lock_cmp_fn for sk_peer_lock.

I'll do so in v2, so

pw-bot: cr


> -	sk->sk_peer_pid  = get_pid(peersk->sk_peer_pid);
> +	spin_lock(&peersk->sk_peer_lock);
> +	sk->sk_peer_pid = get_pid(peersk->sk_peer_pid);
>  	sk->sk_peer_cred = get_cred(peersk->sk_peer_cred);
> -
> -	spin_unlock(&sk->sk_peer_lock);
>  	spin_unlock(&peersk->sk_peer_lock);
> -
> -	put_pid(old_pid);
> -	put_cred(old_cred);
>  }
>  
>  static int unix_listen(struct socket *sock, int backlog)
> -- 
> 2.30.2
>