general protection fault in x25_connect

[syzbot <syzbot+2fde26e61fda58e5f88b@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    8d94a873 Merge branch 'PTP-support-for-the-SJA1105-DSA-dri..
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1065a26aa00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f7a0e5816ab80450
dashboard link: https://syzkaller.appspot.com/bug?extid=2fde26e61fda58e5f88b
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+2fde26e61fda58e5f88b@syzkaller.appspotmail.com

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 21527 Comm: syz-executor.0 Not tainted 5.2.0-rc3+ #21
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:__read_once_size include/linux/compiler.h:194 [inline]
RIP: 0010:arch_atomic_read arch/x86/include/asm/atomic.h:31 [inline]
RIP: 0010:atomic_read include/asm-generic/atomic-instrumented.h:27 [inline]
RIP: 0010:refcount_sub_and_test_checked+0x8e/0x200 lib/refcount.c:182
Code: f3 f3 65 48 8b 04 25 28 00 00 00 48 89 45 d0 31 c0 e8 16 5c 3c fe be  
04 00 00 00 48 89 df e8 49 03 75 fe 48 89 d8 48 c1 e8 03 <42> 0f b6 14 20  
48 89 d8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85
RSP: 0018:ffff8880909dfbc8 EFLAGS: 00010202
RAX: 0000000000000019 RBX: 00000000000000c8 RCX: ffffffff83344bf7
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 00000000000000c8
RBP: ffff8880909dfc60 R08: 1ffffffff12c929c R09: fffffbfff12c929d
R10: fffffbfff12c929c R11: ffffffff896494e3 R12: dffffc0000000000
R13: 0000000000000000 R14: ffff8880909dfc38 R15: 1ffff1101213bf7b
FS:  00007f0de0733700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd127e58000 CR3: 000000008a725000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  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:1834
  __do_sys_connect net/socket.c:1845 [inline]
  __se_sys_connect net/socket.c:1842 [inline]
  __x64_sys_connect+0x73/0xb0 net/socket.c:1842
  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:00007f0de0732c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000459279
RDX: 0000000000000012 RSI: 0000000020000140 RDI: 0000000000000004
RBP: 000000000075c1a0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f0de07336d4
R13: 00000000004bf854 R14: 00000000004d1068 R15: 00000000ffffffff
Modules linked in:
---[ end trace 07098343a5405a38 ]---
RIP: 0010:__read_once_size include/linux/compiler.h:194 [inline]
RIP: 0010:arch_atomic_read arch/x86/include/asm/atomic.h:31 [inline]
RIP: 0010:atomic_read include/asm-generic/atomic-instrumented.h:27 [inline]
RIP: 0010:refcount_sub_and_test_checked+0x8e/0x200 lib/refcount.c:182


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