general protection fault in hfs_find_init

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
10b84daddbec72c6b440216a69de9a9605127f7a (Sat Mar 31 17:59:00 2018 +0000)
Merge branch 'perf-urgent-for-linus' of
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
syzbot dashboard link:
https://syzkaller.appspot.com/bug?extid=7ca256d0da4af073b2e2

C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5475665407115264
syzkaller reproducer:
https://syzkaller.appspot.com/x/repro.syz?id=5340549896732672
Raw console output:
https://syzkaller.appspot.com/x/log.txt?id=5639884018548736
Kernel config:
https://syzkaller.appspot.com/x/.config?id=-2760467897697295172
compiler: gcc (GCC) 7.1.1 20170620

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Reported-by: syzbot+7ca256...@syzkaller.appspotmail.com
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hfs: unable to locate alternate MDB
hfs: continuing without an alternate MDB
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
(ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 4467 Comm: syzkaller143800 Not tainted 4.16.0-rc7+ #9
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:hfs_find_init+0x76/0x180 fs/hfs/bfind.c:21
RSP: 0018:ffff8801acbe6bd0 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffff8801acbe6c58 RCX: ffffffff820dd056
RDX: 0000000000000008 RSI: ffff8801acbe6c58 RDI: ffff8801acbe6c70
RBP: ffff8801acbe6bf0 R08: ffffffff820f0430 R09: 0000000000000000
R10: ffff8801acbe6ce0 R11: ffffed003597cc6f R12: 0000000000000000
R13: 0000000000000040 R14: ffff8801ad972080 R15: ffff8801acbe6cb8
FS: 0000000001e2d880(0000) GS:ffff8801db000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe9b457a9b8 CR3: 00000001ac89e004 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
hfs_ext_read_extent+0x197/0xb50 fs/hfs/extent.c:196
hfs_get_block+0x4cc/0x7a0 fs/hfs/extent.c:360
block_read_full_page+0x293/0x990 fs/buffer.c:2259
hfs_readpage+0x1c/0x20 fs/hfs/inode.c:38
do_read_cache_page+0x637/0x11b0 mm/filemap.c:2804
read_cache_page+0x61/0x80 mm/filemap.c:2892
read_mapping_page include/linux/pagemap.h:402 [inline]
hfs_btree_open+0x536/0x1630 fs/hfs/btree.c:78
hfs_mdb_get+0x16ab/0x22c0 fs/hfs/mdb.c:193
hfs_fill_super+0x1023/0x1930 fs/hfs/super.c:413
mount_bdev+0x2b7/0x370 fs/super.c:1119
hfs_mount+0x34/0x40 fs/hfs/super.c:463
mount_fs+0x66/0x2d0 fs/super.c:1222
vfs_kern_mount.part.26+0xc6/0x4a0 fs/namespace.c:1037
vfs_kern_mount fs/namespace.c:2509 [inline]
do_new_mount fs/namespace.c:2512 [inline]
do_mount+0xea4/0x2bb0 fs/namespace.c:2842
SYSC_mount fs/namespace.c:3058 [inline]
SyS_mount+0xab/0x120 fs/namespace.c:3035
do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x442daa
RSP: 002b:00007ffd0232faf8 EFLAGS: 00000297 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000020000258 RCX: 0000000000442daa
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007ffd0232fb00
RBP: 0000000000000003 R08: 00000000200001c0 R09: 000000000000000a
R10: 0000000000000000 R11: 0000000000000297 R12: 0000000000000004
R13: 0000000000401ca0 R14: 0000000000000000 R15: 0000000000000000
Code: c1 ea 03 80 3c 02 00 0f 85 f2 00 00 00 4d 8d 6c 24 40 48 b8 00 00 00
00 00 fc ff df 48 c7 43 18 00 00 00 00 4c 89 ea 48 c1 ea 03 <0f> b6 04 02
84 c0 74 08 3c 03 0f 8e b0 00 00 00 41 8b 44 24 40
RIP: hfs_find_init+0x76/0x180 fs/hfs/bfind.c:21 RSP: ffff8801acbe6bd0
---[ end trace 5617fd7a7b5bfbcd ]---


---
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[syzbot]

[syzbot]

syzbot suspects this issue was fixed by commit:

commit 42b0ef01e6b5e9c77b383d32c25a0ec2a735d08a
Author: Arnd Bergmann <ar...@arndb.de>
Date: Fri Jul 11 08:46:51 2025 +0000

block: fix FS_IOC_GETLBMD_CAP parsing in blkdev_common_ioctl()

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=17c0d312580000
start commit: ee88bddf7f2f Merge tag 'bpf-fixes' of git://git.kernel.org..
git tree: upstream
kernel config: https://syzkaller.appspot.com/x/.config?x=28cc6f051378bb16
dashboard link: https://syzkaller.appspot.com/bug?extid=7ca256d0da4af073b2e2
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1026b182580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=159e0f0c580000

If the result looks correct, please mark the issue as fixed by replying with:

#syz fix: block: fix FS_IOC_GETLBMD_CAP parsing in blkdev_common_ioctl()

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

[Arnd Bergmann]

On Sat, Sep 6, 2025, at 07:56, syzbot wrote:
> syzbot suspects this issue was fixed by commit:
>
> commit 42b0ef01e6b5e9c77b383d32c25a0ec2a735d08a
> Author: Arnd Bergmann <ar...@arndb.de>
> Date: Fri Jul 11 08:46:51 2025 +0000
>
> block: fix FS_IOC_GETLBMD_CAP parsing in blkdev_common_ioctl()
>
> bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=17c0d312580000
> start commit: ee88bddf7f2f Merge tag 'bpf-fixes' of git://git.kernel.org..
> git tree: upstream
> kernel config: https://syzkaller.appspot.com/x/.config?x=28cc6f051378bb16
> dashboard link: https://syzkaller.appspot.com/bug?extid=7ca256d0da4af073b2e2
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1026b182580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=159e0f0c580000

I took a look and concluded that my patch is unlikely to have
fixed the issue, because:

- my patch was wrong and needed another fixup on top
- the reproducer and kernel log show no reference to ioctl() calls,
so they do not directly interact with the code I changed.

It is possible that my patch is hiding the root cause for the problem,
if part of the reproducer relies on a prior call to ioctl() on a block
device and this ioctl was broken by my patch. This still sounds like a
long shot though, and my first guess would be that the bisection
went wrong, possibly by running into more than one issue, or an
unreliable reproducer.

Arnd