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SubjectKASAN: null-ptr-deref Read in x25_connect

syzbot found the following crash on:

HEAD commit: f4cfcfbd net: dsa: sja1105: Fix link speed not working at ..
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_read
include/asm-generic/atomic-instrumented.h:26 [inline]
BUG: KASAN: null-ptr-deref in refcount_sub_and_test_checked+0x87/0x200
Read of size 4 at addr 00000000000000c8 by task syz-executor.2/16959

CPU: 0 PID: 16959 Comm: syz-executor.2 Not tainted 5.2.0-rc2+ #40
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+0x172/0x1f0 lib/dump_stack.c:113
__kasan_report.cold+0x5/0x40 mm/kasan/report.c:321
kasan_report+0x12/0x20 mm/kasan/common.c:614
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x123/0x190 mm/kasan/generic.c:191
kasan_check_read+0x11/0x20 mm/kasan/common.c:94
atomic_read include/asm-generic/atomic-instrumented.h:26 [inline]
refcount_sub_and_test_checked+0x87/0x200 lib/refcount.c:182
refcount_dec_and_test_checked+0x1b/0x20 lib/refcount.c:220
x25_neigh_put include/net/x25.h:252 [inline]
x25_connect+0x8d8/0xea0 net/x25/af_x25.c:820
__sys_connect+0x264/0x330 net/socket.c:1840
__do_sys_connect net/socket.c:1851 [inline]
__se_sys_connect net/socket.c:1848 [inline]
__x64_sys_connect+0x73/0xb0 net/socket.c:1848
do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
RIP: 0033:0x459279
Code: fd b7 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 b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f09776b4c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000459279
RDX: 0000000000000012 RSI: 0000000020000280 RDI: 0000000000000004
RBP: 000000000075bfc0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f09776b56d4
R13: 00000000004bf854 R14: 00000000004d0e08 R15: 00000000ffffffff

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