[syzbot] BUG: unable to handle kernel NULL pointer dereference in __lookup_slow (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    d93a0d43 Merge tag 'block-5.12-2021-04-02' of git://git.ke..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16519431d00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=71a75beb62b62a34
dashboard link: https://syzkaller.appspot.com/bug?extid=11c49ce9d4e7896f3406
compiler:       Debian clang version 11.0.1-2

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+11c49ce9d4e7896f3406@xxxxxxxxxxxxxxxxxxxxxxxxx

REISERFS (device loop4): Using r5 hash to sort names
BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD 6bb82067 P4D 6bb82067 PUD 6bb81067 PMD 0 
Oops: 0010 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 11072 Comm: syz-executor.4 Not tainted 5.12.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:0x0
Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
RSP: 0018:ffffc90008f8fa20 EFLAGS: 00010246
RAX: 1ffffffff13872e8 RBX: dffffc0000000000 RCX: 0000000000040000
RDX: 0000000000000000 RSI: ffff88802e9d9490 RDI: ffff88807f140190
RBP: ffffffff89c39740 R08: ffffffff81c9d4de R09: fffffbfff200a946
R10: fffffbfff200a946 R11: 0000000000000000 R12: 0000000000000000
R13: ffff88807f140190 R14: 1ffff11005d3b292 R15: ffff88802e9d9490
FS:  00007f894af88700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000006bb83000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __lookup_slow+0x240/0x370 fs/namei.c:1626
 lookup_one_len+0x10e/0x200 fs/namei.c:2649
 reiserfs_lookup_privroot+0x85/0x1e0 fs/reiserfs/xattr.c:980
 reiserfs_fill_super+0x2a69/0x3160 fs/reiserfs/super.c:2176
 mount_bdev+0x26c/0x3a0 fs/super.c:1367
 legacy_get_tree+0xea/0x180 fs/fs_context.c:592
 vfs_get_tree+0x86/0x270 fs/super.c:1497
 do_new_mount fs/namespace.c:2903 [inline]
 path_mount+0x188a/0x29a0 fs/namespace.c:3233
 do_mount fs/namespace.c:3246 [inline]
 __do_sys_mount fs/namespace.c:3454 [inline]
 __se_sys_mount+0x28c/0x320 fs/namespace.c:3431
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x46797a
Code: 48 c7 c2 bc ff ff ff f7 d8 64 89 02 b8 ff ff ff ff eb d2 e8 b8 04 00 00 0f 1f 84 00 00 00 00 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f894af87fa8 EFLAGS: 00000206 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000020000200 RCX: 000000000046797a
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007f894af88000
RBP: 00007f894af88040 R08: 00007f894af88040 R09: 0000000020000000
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000020000000
R13: 0000000020000100 R14: 00007f894af88000 R15: 0000000020011500
Modules linked in:
CR2: 0000000000000000
---[ end trace a1b8dbb111baf993 ]---
RIP: 0010:0x0
Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
RSP: 0018:ffffc90008f8fa20 EFLAGS: 00010246
RAX: 1ffffffff13872e8 RBX: dffffc0000000000 RCX: 0000000000040000
RDX: 0000000000000000 RSI: ffff88802e9d9490 RDI: ffff88807f140190
RBP: ffffffff89c39740 R08: ffffffff81c9d4de R09: fffffbfff200a946
R10: fffffbfff200a946 R11: 0000000000000000 R12: 0000000000000000
R13: ffff88807f140190 R14: 1ffff11005d3b292 R15: ffff88802e9d9490
FS:  00007f894af88700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000006bb83000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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