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SubjectKASAN: use-after-free Read in x25_write_internal

syzbot found the following crash on:

HEAD commit: 7418e6520f22 isdn: hisax: hfc_pci: Fix a possible concurre..
git tree: net
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 8.0.1 20180413 (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:

BUG: KASAN: use-after-free in x25_write_internal+0xded/0xef0
Read of size 4 at addr ffff8880923f6bdc by task syz-executor1/30378

CPU: 0 PID: 30378 Comm: syz-executor1 Not tainted 4.20.0+ #252
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1d3/0x2c6 lib/dump_stack.c:113
print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
__asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
x25_write_internal+0xded/0xef0 net/x25/x25_subr.c:173
x25_release+0x1f8/0x350 net/x25/af_x25.c:660
__sock_release+0xd7/0x250 net/socket.c:579
sock_close+0x19/0x20 net/socket.c:1141
__fput+0x385/0xa30 fs/file_table.c:278
____fput+0x15/0x20 fs/file_table.c:309
task_work_run+0x1e8/0x2a0 kernel/task_work.c:113
get_signal+0x1558/0x1980 kernel/signal.c:2347
do_signal+0x9c/0x21c0 arch/x86/kernel/signal.c:816
exit_to_usermode_loop+0x2e5/0x380 arch/x86/entry/common.c:162
prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
RIP: 0033:0x4579b9
Code: fd b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 0f 83 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f959bdb5c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: fffffffffffffe00 RBX: 0000000000000003 RCX: 00000000004579b9
RDX: 0000000000000012 RSI: 0000000020000080 RDI: 0000000000000005
RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f959bdb66d4
R13: 00000000004be22e R14: 00000000004ce488 R15: 00000000ffffffff

Allocated by task 12427:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:553
kmem_cache_alloc_trace+0x152/0x750 mm/slab.c:3620
kmalloc include/linux/slab.h:546 [inline]
x25_link_device_up+0xb2/0x500 net/x25/x25_link.c:249
x25_device_event+0x116/0x2b0 net/x25/af_x25.c:242
notifier_call_chain+0x17e/0x380 kernel/notifier.c:93
__raw_notifier_call_chain kernel/notifier.c:394 [inline]
raw_notifier_call_chain+0x2d/0x40 kernel/notifier.c:401
call_netdevice_notifiers_info+0x3f/0x90 net/core/dev.c:1739
call_netdevice_notifiers_extack net/core/dev.c:1751 [inline]
call_netdevice_notifiers net/core/dev.c:1765 [inline]
__dev_notify_flags+0x17a/0x480 net/core/dev.c:7605
dev_change_flags+0x109/0x160 net/core/dev.c:7643
dev_ifsioc+0x7da/0xa80 net/core/dev_ioctl.c:237
dev_ioctl+0x1b5/0xcc0 net/core/dev_ioctl.c:488
sock_do_ioctl+0x1f6/0x420 net/socket.c:973
sock_ioctl+0x313/0x690 net/socket.c:1074
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:509 [inline]
do_vfs_ioctl+0x1de/0x1790 fs/ioctl.c:696
ksys_ioctl+0xa9/0xd0 fs/ioctl.c:713
__do_sys_ioctl fs/ioctl.c:720 [inline]
__se_sys_ioctl fs/ioctl.c:718 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290

Freed by task 30378:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
__cache_free mm/slab.c:3498 [inline]
kfree+0xcf/0x230 mm/slab.c:3817
x25_neigh_put include/net/x25.h:253 [inline]
x25_connect+0xc67/0x1050 net/x25/af_x25.c:823
__sys_connect+0x37d/0x4c0 net/socket.c:1664
__do_sys_connect net/socket.c:1675 [inline]
__se_sys_connect net/socket.c:1672 [inline]
__x64_sys_connect+0x73/0xb0 net/socket.c:1672
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290

The buggy address belongs to the object at ffff8880923f6bc0
which belongs to the cache kmalloc-256 of size 256
The buggy address is located 28 bytes inside of
256-byte region [ffff8880923f6bc0, ffff8880923f6cc0)
The buggy address belongs to the page:
page:ffffea000248fd80 count:1 mapcount:0 mapping:ffff88812c3f07c0 index:0x0
flags: 0x1fffc0000000200(slab)
raw: 01fffc0000000200 ffffea00023d4748 ffffea000267af08 ffff88812c3f07c0
raw: 0000000000000000 ffff8880923f6080 000000010000000c 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8880923f6a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880923f6b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff8880923f6b80: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
ffff8880923f6c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880923f6c80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc

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