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Subjectgeneral protection fault in refcount_sub_and_test_checked
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Hello,

syzbot found the following crash on:

HEAD commit: 35f861e3 net: bridge: use netif_is_bridge_port()
git tree: net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10a188ab200000
kernel config: https://syzkaller.appspot.com/x/.config?x=8e9bc94c16d346a6
dashboard link: https://syzkaller.appspot.com/bug?extid=31b6a0f5d6d5c3b75948
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+31b6a0f5d6d5c3b75948@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: 12245 Comm: syz-executor.4 Not tainted 5.1.0-rc2+ #115
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:__read_once_size include/linux/compiler.h:193 [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 36 8a 40 fe be
04 00 00 00 48 89 df e8 29 65 78 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:ffff88808d0bfbc8 EFLAGS: 00010202
RAX: 0000000000000019 RBX: 00000000000000c8 RCX: ffffffff832fe757
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 00000000000000c8
RBP: ffff88808d0bfc60 R08: 1ffffffff12853c4 R09: fffffbfff12853c5
R10: fffffbfff12853c4 R11: ffffffff89429e23 R12: dffffc0000000000
R13: 0000000000000000 R14: ffff88808d0bfc38 R15: 1ffff11011a17f7b
FS: 00007f4fa8af9700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000680c78 CR3: 0000000090024000 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:825
__sys_connect+0x266/0x330 net/socket.c:1808
__do_sys_connect net/socket.c:1819 [inline]
__se_sys_connect net/socket.c:1816 [inline]
__x64_sys_connect+0x73/0xb0 net/socket.c:1816
do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x458209
Code: ad b8 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 b8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f4fa8af8c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000458209
RDX: 0000000000000012 RSI: 00000000200000c0 RDI: 0000000000000005
RBP: 000000000073c220 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f4fa8af96d4
R13: 00000000004be6b3 R14: 00000000004cf2f0 R15: 00000000ffffffff
Modules linked in:
---[ end trace 98127f094cb116c1 ]---
RIP: 0010:__read_once_size include/linux/compiler.h:193 [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


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

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