Subject: Re: [PATCH net] bpf, skmsg: fix NULL pointer dereference in sk_psock_skb_ingress_enqueue

From: John Fastabend <[email-masked]>
Date: Thu, 04 Apr 2024 21:45:48 -0700

Jason Xing wrote:
> Hello John,
> 
> On Thu, Apr 4, 2024 at 9:01 AM John Fastabend <[email-masked]> wrote:
> >
> > Jason Xing wrote:
> > > From: Jason Xing <[email-masked]>
> > >
> > > Fix NULL pointer data-races in sk_psock_skb_ingress_enqueue() which
> > > syzbot reported [1].
> > >
> > > [1]
> > > BUG: KCSAN: data-race in sk_psock_drop / sk_psock_skb_ingress_enqueue
> > >
> > > write to 0xffff88814b3278b8 of 8 bytes by task 10724 on cpu 1:
> > >  sk_psock_stop_verdict net/core/skmsg.c:1257 [inline]
> > >  sk_psock_drop+0x13e/0x1f0 net/core/skmsg.c:843
> > >  sk_psock_put include/linux/skmsg.h:459 [inline]
> > >  sock_map_close+0x1a7/0x260 net/core/sock_map.c:1648
> > >  unix_release+0x4b/0x80 net/unix/af_unix.c:1048
> > >  __sock_release net/socket.c:659 [inline]
> > >  sock_close+0x68/0x150 net/socket.c:1421
> > >  __fput+0x2c1/0x660 fs/file_table.c:422
> > >  __fput_sync+0x44/0x60 fs/file_table.c:507
> > >  __do_sys_close fs/open.c:1556 [inline]
> > >  __se_sys_close+0x101/0x1b0 fs/open.c:1541
> > >  __x64_sys_close+0x1f/0x30 fs/open.c:1541
> > >  do_syscall_64+0xd3/0x1d0
> > >  entry_SYSCALL_64_after_hwframe+0x6d/0x75
> > >
> > > read to 0xffff88814b3278b8 of 8 bytes by task 10713 on cpu 0:
> > >  sk_psock_data_ready include/linux/skmsg.h:464 [inline]
> > >  sk_psock_skb_ingress_enqueue+0x32d/0x390 net/core/skmsg.c:555
> > >  sk_psock_skb_ingress_self+0x185/0x1e0 net/core/skmsg.c:606
> > >  sk_psock_verdict_apply net/core/skmsg.c:1008 [inline]
> > >  sk_psock_verdict_recv+0x3e4/0x4a0 net/core/skmsg.c:1202
> > >  unix_read_skb net/unix/af_unix.c:2546 [inline]
> > >  unix_stream_read_skb+0x9e/0xf0 net/unix/af_unix.c:2682
> > >  sk_psock_verdict_data_ready+0x77/0x220 net/core/skmsg.c:1223
> > >  unix_stream_sendmsg+0x527/0x860 net/unix/af_unix.c:2339
> > >  sock_sendmsg_nosec net/socket.c:730 [inline]
> > >  __sock_sendmsg+0x140/0x180 net/socket.c:745
> > >  ____sys_sendmsg+0x312/0x410 net/socket.c:2584
> > >  ___sys_sendmsg net/socket.c:2638 [inline]
> > >  __sys_sendmsg+0x1e9/0x280 net/socket.c:2667
> > >  __do_sys_sendmsg net/socket.c:2676 [inline]
> > >  __se_sys_sendmsg net/socket.c:2674 [inline]
> > >  __x64_sys_sendmsg+0x46/0x50 net/socket.c:2674
> > >  do_syscall_64+0xd3/0x1d0
> > >  entry_SYSCALL_64_after_hwframe+0x6d/0x75
> > >
> > > value changed: 0xffffffff83d7feb0 -> 0x0000000000000000
> > >
> > > Reported by Kernel Concurrency Sanitizer on:
> > > CPU: 0 PID: 10713 Comm: syz-executor.4 Tainted: G        W          6.8.0-syzkaller-08951-gfe46a7dd189e #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024
> > >
> > > Prior to this, commit 4cd12c6065df ("bpf, sockmap: Fix NULL pointer
> > > dereference in sk_psock_verdict_data_ready()") fixed one NULL pointer
> > > similarly due to no protection of saved_data_ready. Here is another
> > > different caller causing the same issue because of the same reason. So
> > > we should protect it with sk_callback_lock read lock because the writer
> > > side in the sk_psock_drop() uses "write_lock_bh(&sk->sk_callback_lock);".
> > >
> > > Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> > > Reported-by: [email-masked]
> > > Closes: https://syzkaller.appspot.com/bug?extid=aa8c8ec2538929f18f2d
> > > Signed-off-by: Jason Xing <[email-masked]>
> > > ---
> > >  net/core/skmsg.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > > index 4d75ef9d24bf..67c4c01c5235 100644
> > > --- a/net/core/skmsg.c
> > > +++ b/net/core/skmsg.c
> > > @@ -552,7 +552,9 @@ static int sk_psock_skb_ingress_enqueue(struct sk_buff *skb,
> > >       msg->skb = skb;
> > >
> > >       sk_psock_queue_msg(psock, msg);
> > > +     read_lock_bh(&sk->sk_callback_lock);
> > >       sk_psock_data_ready(sk, psock);
> > > +     read_unlock_bh(&sk->sk_callback_lock);
> > >       return copied;
> > >  }
> >
> > The problem is the check and then usage presumably it is already set
> > to NULL:
> >
> >  static inline void sk_psock_data_ready(struct sock *sk, struct sk_psock *psock)
> >  {
> >         if (psock->saved_data_ready)
> >                 psock->saved_data_ready(sk);
> 
> Yes.
> 
> >
> >
> > I'm thinking we might be able to get away with just a READ_ONCE here with
> > similar WRITE_ONCE on other side. Something like this,
> 
> The simple fix that popped into my mind at the beginning is the same
> as you: adding the READ_ONCE/WRITE_ONCE pair.

Let me know if you want to try doing a patch with the READ_ONCE/WRITE_ONCE
we could push something like that through bpf-next I think. Just needs
some extra thought and testing.