Re: KASAN: use-after-free Read in sctp_association_free

[Eric Biggers]

On Thu, Nov 02, 2017 at 08:07:27PM +0800, Xin Long wrote:
> On Thu, Nov 2, 2017 at 1:55 AM, syzbot
> <bot+df9412138a14678abd73a2b70a57241f63563ed1@xxxxxxxxxxxxxxxxxxxxxxxxx>
> wrote:
> > Hello,
> >
> > syzkaller hit the following crash on
> > 25a5d23b47994cdb451dcd2bc8ac310a1492f71b
> > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
> > compiler: gcc (GCC) 7.1.1 20170620
> > .config is attached
> > Raw console output is attached.
> > C reproducer is attached
> > syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> > for information about syzkaller reproducers
> >
> >
> > ==================================================================
> > BUG: KASAN: use-after-free in sctp_association_free+0x7b7/0x930
> > net/sctp/associola.c:333
> > Read of size 8 at addr ffff8801d21d4720 by task syzkaller504854/3007
> >
> > CPU: 0 PID: 3007 Comm: syzkaller504854 Not tainted 4.14.0-rc6+ #62
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:16 [inline]
> >  dump_stack+0x194/0x257 lib/dump_stack.c:52
> >  print_address_description+0x73/0x250 mm/kasan/report.c:252
> >  kasan_report_error mm/kasan/report.c:351 [inline]
> >  kasan_report+0x25b/0x340 mm/kasan/report.c:409
> >  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
> >  sctp_association_free+0x7b7/0x930 net/sctp/associola.c:333
> asoc  could have been freed by sctp_stop_t1_and_abort or elsewhere
> when waiting for buf without holding sk lock.
> 
> One possible fix:
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index c75acdf..e2ea12a 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -2015,7 +2015,7 @@ static int sctp_sendmsg(struct sock *sk, struct
> msghdr *msg, size_t msg_len)
>         goto out_unlock;
> 
>  out_free:
> -       if (new_asoc)
> +       if (new_asoc && err != -ESRCH)
>                 sctp_association_free(asoc);
>  out_unlock:
>         release_sock(sk);
> @@ -7976,10 +7976,11 @@ static int sctp_wait_for_sndbuf(struct
> sctp_association *asoc, long *timeo_p,
>         for (;;) {
>                 prepare_to_wait_exclusive(&asoc->wait, &wait,
>                                           TASK_INTERRUPTIBLE);
> +               if (asoc->base.dead)
> +                       goto do_dead;
>                 if (!*timeo_p)
>                         goto do_nonblock;
> -               if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING ||
> -                   asoc->base.dead)
> +               if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING)
>                         goto do_error;
>                 if (signal_pending(current))
>                         goto do_interrupted;
> @@ -8004,6 +8005,10 @@ static int sctp_wait_for_sndbuf(struct
> sctp_association *asoc, long *timeo_p,
> 
>         return err;
> 
> +do_dead:
> +       err = -ESRCH;
> +       goto out;
> +
>  do_error:
>         err = -EPIPE;
>         goto out;
> 
> will check for sure before posting. thanks.
> 
> >  sctp_sendmsg+0x2311/0x31f0 net/sctp/socket.c:2011
> >  inet_sendmsg+0x11f/0x5e0 net/ipv4/af_inet.c:762
> >  sock_sendmsg_nosec net/socket.c:633 [inline]
> >  sock_sendmsg+0xca/0x110 net/socket.c:643
> >  SYSC_sendto+0x352/0x5a0 net/socket.c:1750
> >  SyS_sendto+0x40/0x50 net/socket.c:1718
> >  do_syscall_32_irqs_on arch/x86/entry/common.c:329 [inline]
> >  do_fast_syscall_32+0x3f2/0xf05 arch/x86/entry/common.c:391
> >  entry_SYSENTER_compat+0x51/0x60 arch/x86/entry/entry_64_compat.S:124
> > RIP: 0023:0xf7fd2c79
> > RSP: 002b:00000000f5fca1ec EFLAGS: 00000292 ORIG_RAX: 0000000000000171
> > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000020925000
> > RDX: 0000000000000002 RSI: 0000000000000000 RDI: 00000000209e1000
> > RBP: 000000000000001c R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> >

This crash has stopped occurring.  I assume it was fixed by commit ca3af4dd28cff
(thanks Xin!), so let's tell syzbot so that it can continue to report crashes in
the same place:

#syz fix: sctp: do not free asoc when it is already dead in sctp_sendmsg

- Eric