KASAN: null-ptr-deref Read in x25_connect

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    f4cfcfbd net: dsa: sja1105: Fix link speed not working at ..
git tree:       net
console output: https://syzkaller.appspot.com/x/log.txt?x=16815cd2a00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4f721a391cd46ea
dashboard link: https://syzkaller.appspot.com/bug?extid=777a2aab6ffd397407b5
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+777a2aab6ffd397407b5@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
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  
lib/refcount.c:182
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
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
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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