RE: [PATCH bpf] skmsg: Fix invalid last sg check in sk_msg_recvmsg()

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

John Fastabend wrote:
> Liu Jian wrote:
> > In sk_psock_skb_ingress_enqueue function, if the linear area + nr_frags +
> > frag_list of the SKB has NR_MSG_FRAG_IDS blocks in total, skb_to_sgvec
> > will return NR_MSG_FRAG_IDS, then msg->sg.end will be set to
> > NR_MSG_FRAG_IDS, and in addition, (NR_MSG_FRAG_IDS - 1) is set to the last
> > SG of msg. Recv the msg in sk_msg_recvmsg, when i is (NR_MSG_FRAG_IDS - 1),
> > the sk_msg_iter_var_next(i) will change i to 0 (not NR_MSG_FRAG_IDS), the
> > judgment condition "msg_rx->sg.start==msg_rx->sg.end" and
> > "i != msg_rx->sg.end" can not work.
> > 
> > As a result, the processed msg cannot be deleted from ingress_msg list.
> > But the length of all the sge of the msg has changed to 0. Then the next
> > recvmsg syscall will process the msg repeatedly, because the length of sge
> > is 0, the -EFAULT error is always returned.
> > 
> > Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> > Signed-off-by: Liu Jian <liujian56@huawei.com>
> > ---
> >  net/core/skmsg.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > index b0fcd0200e84..a8dbea559c7f 100644
> > --- a/net/core/skmsg.c
> > +++ b/net/core/skmsg.c
> > @@ -462,7 +462,7 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
> >  
> >  			if (copied == len)
> >  				break;
> > -		} while (i != msg_rx->sg.end);
> > +		} while (!sg_is_last(sge));
> >  
> >  		if (unlikely(peek)) {
> >  			msg_rx = sk_psock_next_msg(psock, msg_rx);
> > @@ -472,7 +472,7 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
> >  		}
> >  
> >  		msg_rx->sg.start = i;
> > -		if (!sge->length && msg_rx->sg.start == msg_rx->sg.end) {
> > +		if (!sge->length && sg_is_last(sge)) {
> >  			msg_rx = sk_psock_dequeue_msg(psock);
> >  			kfree_sk_msg(msg_rx);
> >  		}
> > -- 
> > 2.17.1
> > 
> 
> Looks correct to me, but I'll test it tomorrow and add a reviewed-by and
> tested-by then. Thanks!

Still testing but adding ack.

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