net test error: KASAN: null-ptr-deref Write in x25_disconnect

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 37255e7a Merge tag 'batadv-net-for-davem-20200427' of git:..
git tree: net
console output: https://syzkaller.appspot.com/x/log.txt?x=12b2e5d8100000
kernel config: https://syzkaller.appspot.com/x/.config?x=b7a70e992f2f9b68
dashboard link: https://syzkaller.appspot.com/bug?extid=6db548b615e5aeefdce2
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+6db548...@syzkaller.appspotmail.com

can: request_module (can-proto-0) failed.
can: request_module (can-proto-0) failed.
can: request_module (can-proto-0) failed.
==================================================================
BUG: KASAN: null-ptr-deref in atomic_fetch_sub include/asm-generic/atomic-instrumented.h:199 [inline]
BUG: KASAN: null-ptr-deref in refcount_sub_and_test include/linux/refcount.h:266 [inline]
BUG: KASAN: null-ptr-deref in refcount_dec_and_test include/linux/refcount.h:294 [inline]
BUG: KASAN: null-ptr-deref in x25_neigh_put include/net/x25.h:253 [inline]
BUG: KASAN: null-ptr-deref in x25_disconnect+0x253/0x370 net/x25/x25_subr.c:361
Write of size 4 at addr 00000000000000d8 by task syz-fuzzer/7147

CPU: 0 PID: 7147 Comm: syz-fuzzer Not tainted 5.7.0-rc2-syzkaller #0
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+0x188/0x20d lib/dump_stack.c:118
__kasan_report.cold+0x5/0x4d mm/kasan/report.c:515
kasan_report+0x33/0x50 mm/kasan/common.c:625
check_memory_region_inline mm/kasan/generic.c:187 [inline]
check_memory_region+0x141/0x190 mm/kasan/generic.c:193
atomic_fetch_sub include/asm-generic/atomic-instrumented.h:199 [inline]
refcount_sub_and_test include/linux/refcount.h:266 [inline]
refcount_dec_and_test include/linux/refcount.h:294 [inline]
x25_neigh_put include/net/x25.h:253 [inline]
x25_disconnect+0x253/0x370 net/x25/x25_subr.c:361
x25_release+0x345/0x420 net/x25/af_x25.c:665
__sock_release+0xcd/0x280 net/socket.c:605
sock_close+0x18/0x20 net/socket.c:1283
__fput+0x33e/0x880 fs/file_table.c:280
task_work_run+0xf4/0x1b0 kernel/task_work.c:123
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_usermode_loop+0x2fa/0x360 arch/x86/entry/common.c:165
prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
syscall_return_slowpath arch/x86/entry/common.c:279 [inline]
do_syscall_64+0x6b1/0x7d0 arch/x86/entry/common.c:305
entry_SYSCALL_64_after_hwframe+0x49/0xb3
RIP: 0033:0x4afb40
Code: 8b 7c 24 10 48 8b 74 24 18 48 8b 54 24 20 49 c7 c2 00 00 00 00 49 c7 c0 00 00 00 00 49 c7 c1 00 00 00 00 48 8b 44 24 08 0f 05 <48> 3d 01 f0 ff ff 76 20 48 c7 44 24 28 ff ff ff ff 48 c7 44 24 30
RSP: 002b:000000c0001e94f8 EFLAGS: 00000216 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 000000c00002c000 RCX: 00000000004afb40
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 000000c0001e9538 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000216 R12: ffffffffffffffff
R13: 0000000000000164 R14: 0000000000000163 R15: 0000000000000200
==================================================================


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