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

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

Jakub Sitnicki wrote:
> On Fri, Jul 23, 2021 at 08:36 PM CEST, Cong Wang wrote:
> > From: Cong Wang <cong.wang@bytedance.com>
> >
> > 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 <eric.dumazet@gmail.com>
> > 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/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 <jakub@cloudflare.com>

Worth doing the small cleanup pointed out by Jakub but feel free to add
my ack.

Acked-by: John Fastabend <john.fastabend@gmail.com>