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

[Xin Long]

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.

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.