Subject: Re: [Patch bpf-next] unix_bpf: fix a potential deadlock in unix_dgram_bpf_recvmsg()

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 28 Jul 2021 12:12:13 +0200

On Fri, Jul 23, 2021 at 08:36 PM CEST, Cong Wang wrote:
> From: Cong Wang <[email-masked]>
>
> As Eric noticed, __unix_dgram_recvmsg() may acquire u->iolock
> too, so we have to release it before calling this function.
>
> Fixes: 9825d866ce0d ("af_unix: Implement unix_dgram_bpf_recvmsg()")
> Reported-by: Eric Dumazet <[email-masked]>
> Cc: John Fastabend <[email-masked]>
> Cc: Daniel Borkmann <[email-masked]>
> Cc: Jakub Sitnicki <[email-masked]>
> Cc: Lorenz Bauer <[email-masked]>
> Signed-off-by: Cong Wang <[email-masked]>
> ---
>  net/unix/unix_bpf.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
> index db0cda29fb2f..b07cb30e87b1 100644
> --- a/net/unix/unix_bpf.c
> +++ b/net/unix/unix_bpf.c
> @@ -53,8 +53,9 @@ static int unix_dgram_bpf_recvmsg(struct sock *sk, struct msghdr *msg,
>  	mutex_lock(&u->iolock);
>  	if (!skb_queue_empty(&sk->sk_receive_queue) &&
>  	    sk_psock_queue_empty(psock)) {
> -		ret = __unix_dgram_recvmsg(sk, msg, len, flags);
> -		goto out;
> +		mutex_unlock(&u->iolock);
> +		sk_psock_put(sk, psock);
> +		return __unix_dgram_recvmsg(sk, msg, len, flags);
>  	}
>  
>  msg_bytes_ready:
> @@ -68,13 +69,13 @@ static int unix_dgram_bpf_recvmsg(struct sock *sk, struct msghdr *msg,
>  		if (data) {
>  			if (!sk_psock_queue_empty(psock))
>  				goto msg_bytes_ready;
> -			ret = __unix_dgram_recvmsg(sk, msg, len, flags);
> -			goto out;
> +			mutex_unlock(&u->iolock);
> +			sk_psock_put(sk, psock);
> +			return __unix_dgram_recvmsg(sk, msg, len, flags);
>  		}
>  		copied = -EAGAIN;
>  	}
>  	ret = copied;
> -out:
>  	mutex_unlock(&u->iolock);
>  	sk_psock_put(sk, psock);
>  	return ret;

Nit: Can be just `return copied`. `ret` became useless.

Acked-by: Jakub Sitnicki <[email-masked]>