netdev - Re: [PATCH v1 net] af_unix: Clear stale u->oob

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Message-ID: <20240405215408.1007-1-kuniyu@amazon.com>
Date: Fri, 5 Apr 2024 14:54:08 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <kuni1840@...il.com>,
	<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
	<rao.shoaib@...cle.com>,
	<syzbot+7f7f201cc2668a8fd169@...kaller.appspotmail.com>
Subject: Re: [PATCH v1 net] af_unix: Clear stale u->oob_skb.

From: Eric Dumazet <edumazet@...gle.com>
Date: Fri, 5 Apr 2024 23:06:32 +0200
> On Fri, Apr 5, 2024 at 10:42 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > syzkaller started to report deadlock of unix_gc_lock after commit
> > 4090fa373f0e ("af_unix: Replace garbage collection algorithm."), but
> > it just uncovers the bug that has been there since commit 314001f0bf92
> > ("af_unix: Add OOB support").
> >
> > The repro basically does the following.
> >
> >   from socket import *
> >   from array import array
> >
> >   c1, c2 = socketpair(AF_UNIX, SOCK_STREAM)
> >   c1.sendmsg([b'a'], [(SOL_SOCKET, SCM_RIGHTS, array("i", [c2.fileno()]))], MSG_OOB)
> >   c2.recv(1)  # blocked as no normal data in recv queue
> >
> >   c2.close()  # done async and unblock recv()
> >   c1.close()  # done async and trigger GC
> >
> > A socket sends its file descriptor to itself as OOB data and tries to
> > receive normal data, but finally recv() fails due to async close().
> >
> > The problem here is wrong handling of OOB skb in manage_oob().  When
> > recvmsg() is called without MSG_OOB, manage_oob() is called to check
> > if the peeked skb is OOB skb.  In such a case, manage_oob() pops it
> > out of the receive queue but does not clear unix_sock(sk)->oob_skb.
> > This is wrong in terms of uAPI.
> >
> > Let's say we send "hello" with MSG_OOB, and "world" without MSG_OOB.
> > The 'o' is handled as OOB data.  When recv() is called twice without
> > MSG_OOB, the OOB data should be lost.
> >
> >   >>> from socket import *
> >   >>> c1, c2 = socketpair(AF_UNIX, SOCK_STREAM, 0)
> >   >>> c1.send(b'hello', MSG_OOB)  # 'o' is OOB data
> >   5
> >   >>> c1.send(b'world')
> >   5
> >   >>> c2.recv(5)  # OOB data is not received
> >   b'hell'
> >   >>> c2.recv(5)  # OOB date is skippeed
> >   b'world'
> >   >>> c2.recv(5, MSG_OOB)  # This should return an error
> >   b'o'
> >
> > In the same situation, TCP actually returns -EINVAL for the last
> > recv().
> >
> > Also, if we do not clear unix_sk(sk)->oob_skb, unix_poll() always set
> > EPOLLPRI even though the data has passed through by previous recv().
> >
> > To avoid these issues, we must clear unix_sk(sk)->oob_skb when dequeuing
> > it from recv queue.
> >
> > The reason why the old GC did not trigger the deadlock is because the
> > old GC relied on the receive queue to detect the loop.
> >
> > When it is triggered, the socket with OOB data is marked as GC candidate
> > because file refcount == inflight count (1).  However, after traversing
> > all inflight sockets, the socket still has a positive inflight count (1),
> > thus the socket is excluded from candidates.  Then, the old GC lose the
> > chance to garbage-collect the socket.
> >
> > With the old GC, the repro continues to create true garbage that will
> > never be freed nor detected by kmemleak as it's linked to the global
> > inflight list.  That's why we couldn't even notice the issue.
> >
> > Fixes: 314001f0bf92 ("af_unix: Add OOB support")
> > Reported-by: syzbot+7f7f201cc2668a8fd169@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=7f7f201cc2668a8fd169
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > ---
> >  net/unix/af_unix.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> > index 5b41e2321209..8f105cf535be 100644
> > --- a/net/unix/af_unix.c
> > +++ b/net/unix/af_unix.c
> > @@ -2665,7 +2665,9 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
> >                                 }
> >                         } else if (!(flags & MSG_PEEK)) {
> >                                 skb_unlink(skb, &sk->sk_receive_queue);
> > -                               consume_skb(skb);
> > +                               WRITE_ONCE(u->oob_skb, NULL);
> > +                               kfree_skb(skb);
> 
> I dunno, this duplicate kfree_skb() is quite unusual and would deserve
> a comment.
> 
> I would perhaps use
> 
>    if (!WARN_ON_ONCE(skb_unref(skb))
>       kfree_skb(skb);
>

Ah, this is what I wanted..!  Somehow I was wondering if I should use
either kfree_skb() or refcount_dec() directly :S

Will post v2, thanks!