BUG: unable to handle kernel NULL pointer dereference in __lookup_slow

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    7c7ec322 Merge tag 'for-linus' of git://git.kernel.org/pub..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17289773900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=240e2ebab67245c7
dashboard link: https://syzkaller.appspot.com/bug?extid=3db80bbf66b88d68af9d
compiler:       gcc (GCC) 10.1.0-syz 20200507

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3db80bbf66b88d68af9d@xxxxxxxxxxxxxxxxxxxxxxxxx

REISERFS (device loop1): Using r5 hash to sort names
REISERFS (device loop1): using 3.5.x disk format
BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD a7454067 P4D a7454067 PUD 93380067 PMD 0 
Oops: 0010 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 9128 Comm: syz-executor.1 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:0x0
Code: Bad RIP value.
RSP: 0018:ffffc90008bbf910 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: 1ffff92001177f25 RCX: ffffc9000aad7000
RDX: 0000000000000000 RSI: ffff888085c9f330 RDI: ffff88804358f7e0
RBP: ffffffff889c4280 R08: 0000000000000001 R09: ffffffff8d461a7f
R10: 0000000000000000 R11: 000000000005f088 R12: ffff888085c9f330
R13: ffff88804358f7e0 R14: ffffc90008bbfaa0 R15: ffffc90008bbf948
FS:  00007f6bb6cc7700(0000) GS:ffff8880ae400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 00000000a75b9000 CR4: 00000000001526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __lookup_slow+0x24c/0x480 fs/namei.c:1544
 lookup_one_len+0x163/0x190 fs/namei.c:2562
 reiserfs_lookup_privroot+0x92/0x280 fs/reiserfs/xattr.c:972
 reiserfs_fill_super+0x211b/0x2df3 fs/reiserfs/super.c:2176
 mount_bdev+0x32e/0x3f0 fs/super.c:1417
 legacy_get_tree+0x105/0x220 fs/fs_context.c:592
 vfs_get_tree+0x89/0x2f0 fs/super.c:1547
 do_new_mount fs/namespace.c:2875 [inline]
 path_mount+0x1387/0x20a0 fs/namespace.c:3192
 do_mount fs/namespace.c:3205 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount fs/namespace.c:3390 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3390
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x460bca
Code: b8 a6 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 dd 87 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 0f 83 ba 87 fb ff c3 66 0f 1f 84 00 00 00 00 00
RSP: 002b:00007f6bb6cc6a88 EFLAGS: 00000202 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f6bb6cc6b20 RCX: 0000000000460bca
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007f6bb6cc6ae0
RBP: 00007f6bb6cc6ae0 R08: 00007f6bb6cc6b20 R09: 0000000020000000
R10: 0000000000a04850 R11: 0000000000000202 R12: 0000000020000000
R13: 0000000020000100 R14: 0000000020000200 R15: 0000000020000040
Modules linked in:
CR2: 0000000000000000
---[ end trace 79d7e2c3db21cbd3 ]---
RIP: 0010:0x0
Code: Bad RIP value.
RSP: 0018:ffffc90008bbf910 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: 1ffff92001177f25 RCX: ffffc9000aad7000
RDX: 0000000000000000 RSI: ffff888085c9f330 RDI: ffff88804358f7e0
RBP: ffffffff889c4280 R08: 0000000000000001 R09: ffffffff8d461a7f
R10: 0000000000000000 R11: 000000000005f088 R12: ffff888085c9f330
R13: ffff88804358f7e0 R14: ffffc90008bbfaa0 R15: ffffc90008bbf948
FS:  00007f6bb6cc7700(0000) GS:ffff8880ae500000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000558411c291f8 CR3: 00000000a75b9000 CR4: 00000000001526e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
This report 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@xxxxxxxxxxxxxxxx.

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