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Date:   Sat, 10 Mar 2018 08:13:55 -0500
From:   Neil Horman <nhorman@tuxdriver.com>
To:     Xin Long <lucien.xin@gmail.com>
Cc:     syzbot <syzbot+a4e4112c3aff00c8cfd8@syzkaller.appspotmail.com>,
        davem <davem@davemloft.net>, LKML <linux-kernel@vger.kernel.org>,
        linux-sctp@vger.kernel.org, network dev <netdev@vger.kernel.org>,
        syzkaller-bugs@googlegroups.com,
        Vlad Yasevich <vyasevich@gmail.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Subject: Re: KASAN: use-after-free Read in sctp_association_free (2)
Message-ID: <20180310131355.GA8798@hmswarspite.think-freely.org>
References: <00000000000017dfe905670110cf@google.com>
 <20180309220823.GA24848@neilslaptop.think-freely.org>
 <CADvbK_fGkhJeirYwiu9ORmmPPBpqamgO2+vi9Osyd92cUWJBVA@mail.gmail.com>
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On Sat, Mar 10, 2018 at 03:58:04PM +0800, Xin Long wrote:
> On Sat, Mar 10, 2018 at 6:08 AM, Neil Horman <nhorman@tuxdriver.com> wrote:
> > On Fri, Mar 09, 2018 at 12:59:06PM -0800, syzbot wrote:
> >> Hello,
> >>
> >> syzbot hit the following crash on net-next commit
> >> fd372a7a9e5e9d8011a0222d10edd3523abcd3b1 (Thu Mar 8 19:43:48 2018 +0000)
> >> Merge tag 'mlx5-updates-2018-02-28-2' of
> >> git://git.kernel.org/pub/scm/linux/kernel/git/mellanox/linux
> >>
> >> So far this crash happened 2 times on net-next.
> >> C reproducer is attached.
> >> syzkaller reproducer is attached.
> >> Raw console output is attached.
> >> compiler: gcc (GCC) 7.1.1 20170620
> >> .config is attached.
> >>
> >> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> >> Reported-by: syzbot+a4e4112c3aff00c8cfd8@syzkaller.appspotmail.com
> >> It will help syzbot understand when the bug is fixed. See footer for
> >> details.
> >> If you forward the report, please keep this part and the footer.
> >>
> >> IPVS: ftp: loaded support on port[0] = 21
> >> IPVS: ftp: loaded support on port[0] = 21
> >> IPVS: ftp: loaded support on port[0] = 21
> >> IPVS: ftp: loaded support on port[0] = 21
> >> ==================================================================
> >> BUG: KASAN: use-after-free in sctp_association_free+0x7b7/0x930
> >> net/sctp/associola.c:332
> >> Read of size 8 at addr ffff8801d8006ae0 by task syzkaller914861/4202
> >>
> >> CPU: 1 PID: 4202 Comm: syzkaller914861 Not tainted 4.16.0-rc4+ #258
> >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> >> Google 01/01/2011
> >> Call Trace:
> >>  __dump_stack lib/dump_stack.c:17 [inline]
> >>  dump_stack+0x194/0x24d lib/dump_stack.c:53
> >>  print_address_description+0x73/0x250 mm/kasan/report.c:256
> >>  kasan_report_error mm/kasan/report.c:354 [inline]
> >>  kasan_report+0x23c/0x360 mm/kasan/report.c:412
> >>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
> >>  sctp_association_free+0x7b7/0x930 net/sctp/associola.c:332
> >>  sctp_sendmsg+0xc67/0x1a80 net/sctp/socket.c:2075
> >>  inet_sendmsg+0x11f/0x5e0 net/ipv4/af_inet.c:763
> >>  sock_sendmsg_nosec net/socket.c:629 [inline]
> >>  sock_sendmsg+0xca/0x110 net/socket.c:639
> >>  SYSC_sendto+0x361/0x5c0 net/socket.c:1748
> >>  SyS_sendto+0x40/0x50 net/socket.c:1716
> >>  do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
> >>  entry_SYSCALL_64_after_hwframe+0x42/0xb7
> >> RIP: 0033:0x446d09
> >> RSP: 002b:00007f5dbac21da8 EFLAGS: 00000216 ORIG_RAX: 000000000000002c
> >> RAX: ffffffffffffffda RBX: 00000000006e29fc RCX: 0000000000446d09
> >> RDX: 0000000000000001 RSI: 0000000020000340 RDI: 0000000000000003
> >> RBP: 00000000006e29f8 R08: 00000000204d9000 R09: 000000000000001c
> >> R10: 0000000000000000 R11: 0000000000000216 R12: 0000000000000000
> >> R13: 00007fff7b26fb1f R14: 00007f5dbac229c0 R15: 00000000006e2b60
> >>
> > I think we have a corner case with a0ff660058b88d12625a783ce9e5c1371c87951f
> > here.  If a peeloff event happens during a wait for sendbuf space, EPIPE will be
> > returned, and the code path appears to call sctp_association_put twice, leading
> > to the use after free situation.  I'll write a patch this weekend
> Hi, Neil, you're right.
> 
> I didn't expect peeloff can be done on a NEW asoc, as peeloff needs
> assoc_id, which can only be set when connecting has started.
> 
> But I realized that:
> f84af33 sctp: factor out sctp_sendmsg_to_asoc from sctp_sendmsg
> 
> moved sctp_primitive_ASSOCIATE(connecting) before sctp_wait_for_sndbuf
> (snd buffer waiting). It means peeloff can be done on a NEW asoc.
> So you may want to move it back.
> 
I agree with the root cause, but I'm not sure I agree with just moving the
wait_for_sndbuf call back above the call to associate.  I'm not sure I like
relying on placing a call in a spcific order solely to avoid an error condition
that might legitimately occur.  I think would rather check the return code at
the call site for the complete set of conditions for which we should not free
the association.  Something like this:

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 7d3476a4860d..a68846d2b0ef 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -2071,8 +2071,9 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)
 
        /* Send msg to the asoc */
        err = sctp_sendmsg_to_asoc(asoc, msg, msg_len, transport, sinfo);
-       if (err < 0 && err != -ESRCH && new)
-               sctp_association_free(asoc);
+       if ((err != -ESRCH) && (err != -EPIPE))
+               if (err < 0 && new)
+                       sctp_association_free(asoc);
 
 out_unlock:
        release_sock(sk);

Which I think also avoids the noted conflict.

Thoughts?

Neil

> One good thing is the fix shouldn't touch the conflict on
> https://lkml.org/lkml/2018/3/7/1175
> We can fix it right now, I think. But pls double check it before
> submitting the patch. We just can't grow up that fixup for linus
> tree's merge.
> 
> Thanks.
>