general protection fault in path_openat

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    ccaaaf6f Merge tag 'mpx-for-linus' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=115bda4ee00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=879390c6b09ccf66
dashboard link: https://syzkaller.appspot.com/bug?extid=190005201ced78a74ad6
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1674c776e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1565e101e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+190005201ced78a74ad6@xxxxxxxxxxxxxxxxxxxxxxxxx

general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 PID: 3515 Comm: modprobe Not tainted 5.5.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:do_last fs/namei.c:3336 [inline]
RIP: 0010:path_openat+0x281/0x3460 fs/namei.c:3607
Code: ff df 48 c1 ea 03 80 3c 02 00 0f 85 76 29 00 00 48 b8 00 00 00 00 00 fc ff df 49 8b 5c 24 58 48 8d 7b 04 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 28
RSP: 0018:ffffc90003c37a40 EFLAGS: 00010247
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff81c24369
RDX: 0000000000000000 RSI: ffffffff81c24376 RDI: 0000000000000004
RBP: ffffc90003c37b90 R08: ffff88809dca63c0 R09: ffffed1015416ca2
R10: ffffed1015416ca1 R11: ffff8880aa0b6508 R12: ffff8880aa0b64a0
R13: ffff88809dca63c0 R14: ffffc90003c37d98 R15: ffffc90003c37bd0
FS:  00007f70d677c700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000009a184000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 do_filp_open+0x192/0x260 fs/namei.c:3637
 do_sys_openat2+0x5eb/0x7e0 fs/open.c:1149
 do_sys_open+0xf2/0x180 fs/open.c:1165
 ksys_open include/linux/syscalls.h:1386 [inline]
 __do_sys_open fs/open.c:1171 [inline]
 __se_sys_open fs/open.c:1169 [inline]
 __x64_sys_open+0x7e/0xc0 fs/open.c:1169
 do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x7f70d6094120
Code: 48 8b 15 1b 4d 2b 00 f7 d8 64 89 02 83 c8 ff c3 90 90 90 90 90 90 90 90 90 90 83 3d d5 a4 2b 00 00 75 10 b8 02 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 5e 8c 01 00 48 89 04 24
RSP: 002b:00007ffdbe2762c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000002
RAX: ffffffffffffffda RBX: 0000558ff35f3240 RCX: 00007f70d6094120
RDX: 00007ffdbe2764fc RSI: 0000000000080000 RDI: 00007ffdbe2764e0
RBP: 00007f70d6564300 R08: 0000000000000000 R09: 0000558ff35f3219
R10: 0000000000000000 R11: 0000000000000246 R12: 0000558ff35eb210
R13: 0000558ff35f3210 R14: 0000558ff35eb210 R15: 00007ffdbe2764e0
Modules linked in:
---[ end trace bdc3fba9ce9969a3 ]---
RIP: 0010:do_last fs/namei.c:3336 [inline]
RIP: 0010:path_openat+0x281/0x3460 fs/namei.c:3607
Code: ff df 48 c1 ea 03 80 3c 02 00 0f 85 76 29 00 00 48 b8 00 00 00 00 00 fc ff df 49 8b 5c 24 58 48 8d 7b 04 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 28
RSP: 0018:ffffc90003c37a40 EFLAGS: 00010247
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff81c24369
RDX: 0000000000000000 RSI: ffffffff81c24376 RDI: 0000000000000004
RBP: ffffc90003c37b90 R08: ffff88809dca63c0 R09: ffffed1015416ca2
R10: ffffed1015416ca1 R11: ffff8880aa0b6508 R12: ffff8880aa0b64a0
R13: ffff88809dca63c0 R14: ffffc90003c37d98 R15: ffffc90003c37bd0
FS:  00007f70d677c700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fdd12577518 CR3: 000000009a184000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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