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SubjectKASAN: null-ptr-deref Write in x25_connect

syzbot found the following crash on:

HEAD commit: 9bd19c63 net: emulex: benet: indent a Kconfig depends cont..
git tree: net
console output:
kernel config:
dashboard link:
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:

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:253 [inline]
BUG: KASAN: null-ptr-deref in refcount_dec_and_test
include/linux/refcount.h:281 [inline]
BUG: KASAN: null-ptr-deref in x25_neigh_put include/net/x25.h:252 [inline]
BUG: KASAN: null-ptr-deref in x25_connect+0x974/0x1020 net/x25/af_x25.c:820
Write of size 4 at addr 00000000000000c8 by task syz-executor.5/32400

CPU: 1 PID: 32400 Comm: syz-executor.5 Not tainted 5.4.0-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+0x197/0x210 lib/dump_stack.c:118
__kasan_report.cold+0x5/0x41 mm/kasan/report.c:510
kasan_report+0x12/0x20 mm/kasan/common.c:634
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
__kasan_check_write+0x14/0x20 mm/kasan/common.c:98
atomic_fetch_sub include/asm-generic/atomic-instrumented.h:199 [inline]
refcount_sub_and_test include/linux/refcount.h:253 [inline]
refcount_dec_and_test include/linux/refcount.h:281 [inline]
x25_neigh_put include/net/x25.h:252 [inline]
x25_connect+0x974/0x1020 net/x25/af_x25.c:820
__sys_connect_file+0x25d/0x2e0 net/socket.c:1847
__sys_connect+0x51/0x90 net/socket.c:1860
__do_sys_connect net/socket.c:1871 [inline]
__se_sys_connect net/socket.c:1868 [inline]
__x64_sys_connect+0x73/0xb0 net/socket.c:1868
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
RIP: 0033:0x45a679
Code: ad b6 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 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fa58a10ec78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045a679
RDX: 0000000000000012 RSI: 0000000020000000 RDI: 0000000000000004
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fa58a10f6d4
R13: 00000000004c0f1c R14: 00000000004d4088 R15: 00000000ffffffff

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