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

[Dmitry Vyukov]

On Tue, Jan 16, 2018 at 7:29 PM, syzbot
<syzbot+065d0fc357520c8f6039@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> 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: syzbot+065d0fc357520c8f6039@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.


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:
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.

Thanks


> ==================================================================
> 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
>>
>> 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
> ==================================================================
>
>
> ---
> This bug is generated by a dumb bot. It may contain errors.
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