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

From: syzbot <[email-masked]>
Date: Tue, 16 Jan 2018 10:29:02 -0800

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.

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Reported-by: [email-masked]
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==================================================================
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
==================================================================

---
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