general protection fault in relay_close_buf

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    371dd432 Merge branch 'for-5.1-fixes' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=124077dd200000
kernel config:  https://syzkaller.appspot.com/x/.config?x=856fc6d0fbbeede9
dashboard link: https://syzkaller.appspot.com/bug?extid=58320b7171734bf79d26
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+58320b7171734bf79d26@xxxxxxxxxxxxxxxxxxxxxxxxx

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: 0 PID: 6042 Comm: syz-executor.4 Not tainted 5.1.0-rc5+ #76
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:relay_close_buf+0x28/0x150 kernel/relay.c:495
Code: 00 00 55 48 89 e5 41 54 49 89 fc 53 e8 a1 1d ff ff 49 8d bc 24 ac 00  
00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48  
89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 dc
RSP: 0018:ffff8880976d7918 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: dffffc0000000000 RCX: ffffc9000e6c3000
RDX: 0000000020000015 RSI: ffffffff81716f0f RDI: 00000001000000ab
RBP: ffff8880976d7928 R08: ffff88808de7a640 R09: ffffed1012edaefa
R10: ffffed1012edaef9 R11: 0000000000000003 R12: 00000000ffffffff
R13: ffff88809f86a300 R14: 0000000000000007 R15: 00000000ffffffff
FS:  00007f4e560ef700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000002540040 CR3: 00000000a158b000 CR4: 00000000001406f0
Call Trace:
 relay_open kernel/relay.c:612 [inline]
 relay_open+0x619/0x980 kernel/relay.c:563
 do_blk_trace_setup+0x414/0xb90 kernel/trace/blktrace.c:532
 __blk_trace_setup+0xe3/0x190 kernel/trace/blktrace.c:577
 blk_trace_ioctl+0x170/0x300 kernel/trace/blktrace.c:716
 blkdev_ioctl+0x276/0x1d10 block/ioctl.c:591
 block_ioctl+0xee/0x130 fs/block_dev.c:1933
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:509 [inline]
 do_vfs_ioctl+0xd6e/0x1390 fs/ioctl.c:696
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:713
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
 do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x458c29
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:00007f4e560eec78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000458c29
RDX: 0000000020000080 RSI: 00000000c0481273 RDI: 0000000000000005
RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f4e560ef6d4
R13: 00000000004c00f2 R14: 00000000004d24f8 R15: 00000000ffffffff
Modules linked in:
---[ end trace 9ca3bd995bc8baa6 ]---
RIP: 0010:relay_close_buf+0x28/0x150 kernel/relay.c:495
Code: 00 00 55 48 89 e5 41 54 49 89 fc 53 e8 a1 1d ff ff 49 8d bc 24 ac 00  
00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48  
89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 dc
RSP: 0018:ffff8880976d7918 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: dffffc0000000000 RCX: ffffc9000e6c3000
RDX: 0000000020000015 RSI: ffffffff81716f0f RDI: 00000001000000ab
RBP: ffff8880976d7928 R08: ffff88808de7a640 R09: ffffed1012edaefa
R10: ffffed1012edaef9 R11: 0000000000000003 R12: 00000000ffffffff
R13: ffff88809f86a300 R14: 0000000000000007 R15: 00000000ffffffff
FS:  00007f4e560ef700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000002540040 CR3: 00000000a158b000 CR4: 00000000001406f0


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