KASAN: use-after-free Read in pppol2tp_session_sock_hold

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 01fd1694 Linux 4.14.141
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=15116ff2600000
kernel config: https://syzkaller.appspot.com/x/.config?x=62c9b69e1b2adda9
dashboard link: https://syzkaller.appspot.com/bug?extid=2832d48dc9e984d91d71
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+2832d4...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in __read_once_size include/linux/compiler.h:183
[inline]
BUG: KASAN: use-after-free in atomic_read arch/x86/include/asm/atomic.h:27
[inline]
BUG: KASAN: use-after-free in refcount_inc_not_zero+0xd3/0xe0
lib/refcount.c:120
Read of size 4 at addr ffff888067d23580 by task kworker/u4:0/5

CPU: 1 PID: 5 Comm: kworker/u4:0 Not tainted 4.14.141 #37
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: l2tp l2tp_tunnel_del_work
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x138/0x197 lib/dump_stack.c:53
print_address_description.cold+0x7c/0x1dc mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report mm/kasan/report.c:409 [inline]
kasan_report.cold+0xa9/0x2af mm/kasan/report.c:393
__asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:429
__read_once_size include/linux/compiler.h:183 [inline]
atomic_read arch/x86/include/asm/atomic.h:27 [inline]
refcount_inc_not_zero+0xd3/0xe0 lib/refcount.c:120
refcount_inc+0x16/0x40 lib/refcount.c:153
sock_hold include/net/sock.h:619 [inline]
pppol2tp_session_sock_hold+0x48/0x60 net/l2tp/l2tp_ppp.c:263
l2tp_tunnel_closeall+0x1ef/0x380 net/l2tp/l2tp_core.c:1332
l2tp_tunnel_del_work+0x64/0x410 net/l2tp/l2tp_core.c:1380
process_one_work+0x863/0x1600 kernel/workqueue.c:2114
worker_thread+0x5d9/0x1050 kernel/workqueue.c:2248
kthread+0x319/0x430 kernel/kthread.c:232
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404

Allocated by task 3527:
save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
save_stack+0x45/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc mm/kasan/kasan.c:551 [inline]
kasan_kmalloc+0xce/0xf0 mm/kasan/kasan.c:529
kasan_slab_alloc+0xf/0x20 mm/kasan/kasan.c:489
kmem_cache_alloc+0x12e/0x780 mm/slab.c:3552
getname_flags fs/namei.c:138 [inline]
getname_flags+0xcb/0x580 fs/namei.c:128
user_path_at_empty+0x2f/0x50 fs/namei.c:2630
user_path_at include/linux/namei.h:57 [inline]
vfs_statx+0xcd/0x160 fs/stat.c:185
vfs_stat include/linux/fs.h:3060 [inline]
SYSC_newstat+0x95/0x100 fs/stat.c:337
SyS_newstat+0x1e/0x30 fs/stat.c:333
do_syscall_64+0x1e8/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 3527:
save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
save_stack+0x45/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_slab_free+0x75/0xc0 mm/kasan/kasan.c:524
__cache_free mm/slab.c:3496 [inline]
kmem_cache_free+0x83/0x2b0 mm/slab.c:3758
putname+0xdb/0x120 fs/namei.c:259
filename_lookup+0x23a/0x380 fs/namei.c:2385
user_path_at_empty+0x43/0x50 fs/namei.c:2630
user_path_at include/linux/namei.h:57 [inline]
vfs_statx+0xcd/0x160 fs/stat.c:185
vfs_stat include/linux/fs.h:3060 [inline]
SYSC_newstat+0x95/0x100 fs/stat.c:337
SyS_newstat+0x1e/0x30 fs/stat.c:333
do_syscall_64+0x1e8/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the object at ffff888067d22e80
which belongs to the cache names_cache of size 4096
The buggy address is located 1792 bytes inside of
4096-byte region [ffff888067d22e80, ffff888067d23e80)
The buggy address belongs to the page:
page:ffffea00019f4880 count:1 mapcount:0 mapping:ffff888067d22e80 index:0x0
compound_mapcount: 0
flags: 0x1fffc0000008100(slab|head)
raw: 01fffc0000008100 ffff888067d22e80 0000000000000000 0000000100000001
raw: ffffea00022ea120 ffffea00019af6a0 ffff8880aa9e0cc0 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888067d23480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888067d23500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff888067d23580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
kobject: 'loop3' (ffff8880a4afc160): fill_kobj_path: path
= '/devices/virtual/block/loop3'
^
ffff888067d23600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888067d23680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.