Re: KASAN: use-after-free Read in l2tp_session_create

On Fri, Jul 20, 2018 at 9:53 AM, James Chapman [off-list ref] wrote:

On 18/07/18 12:00, Dmitry Vyukov wrote:

quoted

On Tue, Jan 16, 2018 at 7:29 PM, syzbot
[off-list ref] wrote:

quoted

Hello,

syzkaller hit the following crash on
a8750ddca918032d6349adbf9a4b6555e7db20da
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.
Unfortunately, I don't have any reproducer for this bug yet.


IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: <redacted>
It will help syzbot understand when the bug is fixed. See footer for
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If you forward the report, please keep this part and the footer.

James,

Did you fix this? You asked syzbot to test a fix for this bug some time ago.
If yes, did you include the Reported-by tag in the commit? This bug is
still considered open by syzbot. But it stopped happening ~4 months
ago:

Yes, I think this has been fixed now. I think it was fixed by
Guillaume's 6b9f34239b00e6956a267abed2bc559ede556ad6 that was actually
to fix another syzbot bug fbeeb5c3b538e8545644 which looks similar to
this one.

quoted

https://syzkaller.appspot.com/bug?id=6fed0854381422329e78d7e16fb9cf4af8c9aef1
We are also seeing these crashes in 4.4 and 4.9, it would be good to
backport the fix.

It looks like 6b9f34239b00e6956a267abed2bc559ede556ad6 hasn't made it to
4.9 or 4.4.

Thanks for the update!

Let's tell syzbot that this is fixed:

#syz fix: l2tp: fix races in tunnel creation

Greg H: so this is probably the patch we need.

+Greg KH: I think we need this in stable, we hit this in both 4.4 and 4.9.

quoted

quoted

==================================================================
BUG: KASAN: use-after-free in l2tp_session_create+0xa6d/0xc60
net/l2tp/l2tp_core.c:1757
Read of size 4 at addr ffff8801d80ad868 by task syz-executor3/5462

CPU: 0 PID: 5462 Comm: syz-executor3 Not tainted 4.15.0-rc8+ #263
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+0x194/0x257 lib/dump_stack.c:53
 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_load4_noabort+0x14/0x20 mm/kasan/report.c:429
 l2tp_session_create+0xa6d/0xc60 net/l2tp/l2tp_core.c:1757
 pppol2tp_connect+0xed7/0x1dd0 net/l2tp/l2tp_ppp.c:748
 SYSC_connect+0x213/0x4a0 net/socket.c:1621
 SyS_connect+0x24/0x30 net/socket.c:1602
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x452df9
RSP: 002b:00007f93ec47fc58 EFLAGS: 00000212 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 000000000071bea0 RCX: 0000000000452df9
RDX: 000000000000002e RSI: 00000000205fafd2 RDI: 0000000000000018
RBP: 00000000000005a9 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000212 R12: 00000000006f6878
R13: 00000000ffffffff R14: 00007f93ec4806d4 R15: 0000000000000000

Allocated by task 5462:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
 kmem_cache_alloc_trace+0x136/0x750 mm/slab.c:3610
 kmalloc include/linux/slab.h:499 [inline]
 kzalloc include/linux/slab.h:688 [inline]
 l2tp_tunnel_create+0x5e1/0x17f0 net/l2tp/l2tp_core.c:1554
 pppol2tp_connect+0x14b7/0x1dd0 net/l2tp/l2tp_ppp.c:707
 SYSC_connect+0x213/0x4a0 net/socket.c:1621
 SyS_connect+0x24/0x30 net/socket.c:1602
 entry_SYSCALL_64_fastpath+0x29/0xa0

Freed by task 5484:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3488 [inline]
 kfree+0xd6/0x260 mm/slab.c:3803
 __rcu_reclaim kernel/rcu/rcu.h:190 [inline]
 rcu_do_batch kernel/rcu/tree.c:2758 [inline]
 invoke_rcu_callbacks kernel/rcu/tree.c:3012 [inline]
 __rcu_process_callbacks kernel/rcu/tree.c:2979 [inline]
 rcu_process_callbacks+0xe94/0x17f0 kernel/rcu/tree.c:2996
 __do_softirq+0x2d7/0xb85 kernel/softirq.c:285

The buggy address belongs to the object at ffff8801d80ad780
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 232 bytes inside of
 512-byte region [ffff8801d80ad780, ffff8801d80ad980)
The buggy address belongs to the page:
page:ffffea0007602b40 count:1 mapcount:0 mapping:ffff8801d80ad000 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801d80ad000 0000000000000000 0000000100000006
raw: ffffea00070e8760 ffffea00070f8ca0 ffff8801dac00940 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801d80ad700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801d80ad780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

quoted

ffff8801d80ad800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

                                                          ^
 ffff8801d80ad880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801d80ad900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
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