Re: KASAN: use-after-free Read in sctp_association_free (2)

[Neil Horman]

On Mon, Mar 12, 2018 at 04:16:27PM +0800, Xin Long wrote:
> On Sun, Mar 11, 2018 at 3:04 AM, Neil Horman <nhorman@xxxxxxxxxxxxx> wrote:
> > On Sun, Mar 11, 2018 at 12:22:32AM +0800, Xin Long wrote:
> >> On Sat, Mar 10, 2018 at 9:13 PM, Neil Horman <nhorman@xxxxxxxxxxxxx> wrote:
> >> > 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> >> >> 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?
> >> If sctp_association_free is called for general asoc, yes, I agree with you.
> >> But it's actually only for NEW asoc, a special case, not worth a extra check.
> >> 'err != -ESRCH' is already kind of ugly there (I couldn't find a nicer way :D),
> >> I don't hope there will be more like that.
> >>
> > I agree with you on the uglyness aspect of the return code check, but I really
> > don't like the idea of placing the call to wait_for_sndbuf to guarantee a given
> > error code isn't returned, It just feels rickety to me.
> I understand, let's not count this moving back as the official fix
> for this, but only for the compatibility :-)
> 
> Then we can start a new one for improving it later as you said below,
> FYI, we've tried to pass 'new' parameter into sctp_sendmsg_to_asoc
> or use &asoc as the parameter instead. But it seems not good, and
> also sctp_association_free will have to be done in sctp_sendmsg_to_asoc,
> which looks worse.
> 
Yeah, I thought about that, and don't particularly care for it either.  I also
thought about checking the dead flag at the end of send_to_asoc, but that won't
work either, as the caller of wait_for_sndbuf still has a reference.  I think,
for now, you were right the first time, we just need to reposition it.   I'll be
checking the fix with the provided reproducer today.

Neil