BUG: unable to handle kernel NULL pointer dereference in ep_item_poll

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    90368a37fbbe Merge tag 'printk-for-4.18-rc3' of git://git...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14e0dd14400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a63be0c83e84d370
dashboard link: https://syzkaller.appspot.com/bug?extid=57727883dbad76db2ef0
compiler:       gcc (GCC) 8.0.1 20180413 (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+57727883dbad76db2ef0@xxxxxxxxxxxxxxxxxxxxxxxxx

device lo entered promiscuous mode
device lo left promiscuous mode
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 1ccf13067 P4D 1ccf13067 PUD 1bc558067 PMD 0
Oops: 0010 [#1] SMP KASAN
CPU: 1 PID: 13288 Comm: syz-executor0 Not tainted 4.18.0-rc2+ #121
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff8801ce74f568 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8801b2a681c0 RCX: ffffc90001e14000
RDX: ffff8801ce74f7c0 RSI: ffff8801b2a681c0 RDI: ffff8801b819ed00
RBP: ffff8801ce74f610 R08: ffff8801cfc64480 R09: ffffed003b5e46d6
R10: ffffed003b5e46d6 R11: ffff8801daf236b3 R12: ffff8801ce74f7c0
R13: ffff8801b819ed00 R14: ffffffff8818a920 R15: ffff8801cfd33740
FS:  00007f1c1e46f700(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 00000001ca482000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 vfs_poll include/linux/poll.h:86 [inline]
 ep_item_poll.isra.14+0x15c/0x400 fs/eventpoll.c:887
 ep_insert+0x6c1/0x1c20 fs/eventpoll.c:1459
 __do_sys_epoll_ctl fs/eventpoll.c:2113 [inline]
 __se_sys_epoll_ctl fs/eventpoll.c:1999 [inline]
 __x64_sys_epoll_ctl+0xf00/0x10a0 fs/eventpoll.c:1999
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x455a99
Code: 1d ba 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 eb b9 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f1c1e46ec68 EFLAGS: 00000246 ORIG_RAX: 00000000000000e9
RAX: ffffffffffffffda RBX: 00007f1c1e46f6d4 RCX: 0000000000455a99
RDX: 0000000000000013 RSI: 0000000000000001 RDI: 0000000000000014
RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000020000180 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000004bb9c5 R14: 00000000004c8910 R15: 0000000000000000
Modules linked in:
Dumping ftrace buffer:
   (ftrace buffer empty)
CR2: 0000000000000000
---[ end trace 727efc30ae8b7e22 ]---
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff8801ce74f568 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8801b2a681c0 RCX: ffffc90001e14000
RDX: ffff8801ce74f7c0 RSI: ffff8801b2a681c0 RDI: ffff8801b819ed00
RBP: ffff8801ce74f610 R08: ffff8801cfc64480 R09: ffffed003b5e46d6
R10: ffffed003b5e46d6 R11: ffff8801daf236b3 R12: ffff8801ce74f7c0
R13: ffff8801b819ed00 R14: ffffffff8818a920 R15: ffff8801cfd33740
FS:  00007f1c1e46f700(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 00000001ca482000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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