RE: [PATCH bpf-next] skmsg: Fix wrong last sg check in sk_msg_recvmsg()

["liujian (CE)" <liujian56@huawei.com>]

> -----Original Message-----
> From: John Fastabend [mailto:john.fastabend@gmail.com]
> Sent: Tuesday, August 9, 2022 2:58 PM
> To: liujian (CE) <liujian56@huawei.com>; john.fastabend@gmail.com;
> jakub@cloudflare.com; davem@davemloft.net; edumazet@google.com;
> kuba@kernel.org; pabeni@redhat.com; daniel@iogearbox.net;
> andrii@kernel.org; netdev@vger.kernel.org; bpf@vger.kernel.org
> Cc: liujian (CE) <liujian56@huawei.com>
> Subject: RE: [PATCH bpf-next] skmsg: Fix wrong last sg check in
> sk_msg_recvmsg()
> 
> Liu Jian wrote:
> > Fix one kernel NULL pointer dereference as below:
> >
> > [  224.462334] Call Trace:
> > [  224.462394]  __tcp_bpf_recvmsg+0xd3/0x380 [  224.462441]  ?
> > sock_has_perm+0x78/0xa0 [  224.462463]  tcp_bpf_recvmsg+0x12e/0x220 [
> > 224.462494]  inet_recvmsg+0x5b/0xd0 [  224.462534]
> > __sys_recvfrom+0xc8/0x130 [  224.462574]  ?
> > syscall_trace_enter+0x1df/0x2e0 [  224.462606]  ?
> > __do_page_fault+0x2de/0x500 [  224.462635]
> > __x64_sys_recvfrom+0x24/0x30 [  224.462660]  do_syscall_64+0x5d/0x1d0
> > [  224.462709]  entry_SYSCALL_64_after_hwframe+0x65/0xca
> >
> > In commit 7303524e04af ("skmsg: Lose offset info in
> > sk_psock_skb_ingress"), we change last sg check to sg_is_last(), but
> > in sockmap redirection case (without
> > stream_parser/stream_verdict/skb_verdict), we did not mark the end of
> > the scatterlist. Check the sk_msg_alloc, sk_msg_page_add, and
> > bpf_msg_push_data functions, they all do not mark the end of sg. They
> > are expected to use sg.end for end judgment. So the judgment of '(i !=
> msg_rx->sg.end)' is added back here.
> >
> > Fixes: 7303524e04af ("skmsg: Lose offset info in
> > sk_psock_skb_ingress")
> > Signed-off-by: Liu Jian <liujian56@huawei.com>
> > ---
> 
> This is the wrong fixes tag though right? We should have,

I am sorry for this, and will send v2 to update the fix tag.

> 
> 9974d37ea75f0 ("skmsg: Fix invalid last sg check in sk_msg_recvmsg()")
> 
> Fix looks OK though although its not great we have two ways to find the last
> frag now. I'm going to look at getting some better testing in place and then
> see if we can get to just one check.
> 
> Assuming I'm right on the fixes tag please update that.
> 
> >  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
> > 81627892bdd4..385ae23580a5 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 (!sg_is_last(sge));
> > +		} while ((i != msg_rx->sg.end) && !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 && sg_is_last(sge)) {
> > +		if (!sge->length && (i == msg_rx->sg.end || sg_is_last(sge)))
> {
> >  			msg_rx = sk_psock_dequeue_msg(psock);
> >  			kfree_sk_msg(msg_rx);
> >  		}
> > --
> > 2.17.1
> >
>