[syzbot] [bcachefs?] kernel BUG in bch2_btree_path_level_init (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: d69eb204c255 Merge tag 'net-6.17-rc5' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16d01962580000
kernel config: https://syzkaller.appspot.com/x/.config?x=bd9738e00c1bbfb4
dashboard link: https://syzkaller.appspot.com/bug?extid=55c84106264e92ea9ada
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1788c962580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12b0887c580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/efa41ea44c52/disk-d69eb204.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d9ae66f326a4/vmlinux-d69eb204.xz
kernel image: https://storage.googleapis.com/syzbot-assets/967c2253578b/bzImage-d69eb204.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/9e7c59fc7ab3/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+55c841...@syzkaller.appspotmail.com

running recovery passes: check_allocations,check_extents_to_backpointers,check_inodes
bcachefs (loop0): invalid bkey in btree_node btree=inodes level=0: u64s 18 type inode_v3 0:4102:U32_MAX len 0 ver 0: (unpack error)
invalid variable length fields, deleting
bcachefs (loop0): accounting_read... done
bcachefs (loop0): alloc_read... done
bcachefs (loop0): alloc_read... done
bcachefs (loop0): snapshots_read... done
bcachefs (loop0): check_allocations...
------------[ cut here ]------------
kernel BUG at fs/bcachefs/btree_iter.c:633!
Oops: invalid opcode: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 6000 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:bch2_btree_path_level_init+0xa01/0xa20 fs/bcachefs/btree_iter.c:633
Code: fd 90 0f 0b e8 90 05 a2 fd 90 0f 0b e8 88 05 a2 fd 90 0f 0b e8 80 05 a2 fd 90 0f 0b e8 78 05 a2 fd 90 0f 0b e8 70 05 a2 fd 90 <0f> 0b e8 68 05 a2 fd 90 0f 0b e8 60 05 a2 fd 90 0f 0b 66 66 66 66
RSP: 0018:ffffc90003f8e4a0 EFLAGS: 00010293
RAX: ffffffff841c59a0 RBX: ffff88803d33c2d8 RCX: ffff888030d73b80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: dffffc0000000000 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffed1007c6c817 R12: 0000000000000052
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS: 000055557a974500(0000) GS:ffff8881268c1000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fff395c5f24 CR3: 0000000027dc0000 CR4: 00000000003526f0
Call Trace:
<TASK>
btree_path_down fs/bcachefs/btree_iter.c:994 [inline]
bch2_btree_path_traverse_one+0x1367/0x21d0 fs/bcachefs/btree_iter.c:1202
bch2_btree_path_traverse fs/bcachefs/btree_iter.h:250 [inline]
__bch2_btree_iter_peek fs/bcachefs/btree_iter.c:2267 [inline]
bch2_btree_iter_peek_max+0x9e8/0x58f0 fs/bcachefs/btree_iter.c:2384
bch2_btree_iter_peek_max_type fs/bcachefs/btree_iter.h:743 [inline]
bch2_gc_btree fs/bcachefs/btree_gc.c:729 [inline]
bch2_gc_btrees fs/bcachefs/btree_gc.c:792 [inline]
bch2_check_allocations+0x120d/0x57b0 fs/bcachefs/btree_gc.c:1094
bch2_run_recovery_pass fs/bcachefs/recovery_passes.c:484 [inline]
__bch2_run_recovery_passes+0x3bd/0x1060 fs/bcachefs/recovery_passes.c:539
bch2_run_recovery_passes+0x184/0x210 fs/bcachefs/recovery_passes.c:610
bch2_fs_recovery+0x2690/0x3a50 fs/bcachefs/recovery.c:1016
bch2_fs_start+0xaaf/0xda0 fs/bcachefs/super.c:1213
bch2_fs_get_tree+0xb39/0x1520 fs/bcachefs/fs.c:2488
vfs_get_tree+0x8f/0x2b0 fs/super.c:1815
do_new_mount+0x2a2/0x9e0 fs/namespace.c:3808
do_mount fs/namespace.c:4136 [inline]
__do_sys_mount fs/namespace.c:4347 [inline]
__se_sys_mount+0x317/0x410 fs/namespace.c:4324
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fcb57f3038a
Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb a6 e8 de 1a 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc365dc0a8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007ffc365dc130 RCX: 00007fcb57f3038a
RDX: 0000200000000040 RSI: 0000200000004940 RDI: 00007ffc365dc0f0
RBP: 0000200000000040 R08: 00007ffc365dc130 R09: 0000000000004000
R10: 0000000000004000 R11: 0000000000000246 R12: 0000200000004940
R13: 00007ffc365dc0f0 R14: 00000000000049ae R15: 0000200000000180
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:bch2_btree_path_level_init+0xa01/0xa20 fs/bcachefs/btree_iter.c:633
Code: fd 90 0f 0b e8 90 05 a2 fd 90 0f 0b e8 88 05 a2 fd 90 0f 0b e8 80 05 a2 fd 90 0f 0b e8 78 05 a2 fd 90 0f 0b e8 70 05 a2 fd 90 <0f> 0b e8 68 05 a2 fd 90 0f 0b e8 60 05 a2 fd 90 0f 0b 66 66 66 66
RSP: 0018:ffffc90003f8e4a0 EFLAGS: 00010293
RAX: ffffffff841c59a0 RBX: ffff88803d33c2d8 RCX: ffff888030d73b80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: dffffc0000000000 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffed1007c6c817 R12: 0000000000000052
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS: 000055557a974500(0000) GS:ffff8881268c1000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fff395c5f24 CR3: 0000000027dc0000 CR4: 00000000003526f0


---
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[syzbot]

syzbot has bisected this issue to:

commit f11ca2ab18e369a662c2f60d53b8bea46c54e312
Author: Kent Overstreet <kent.ov...@linux.dev>
Date: Thu Nov 28 03:29:54 2024 +0000

bcachefs: Guard against backpointers to unknown btrees

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=1663c962580000
start commit: d69eb204c255 Merge tag 'net-6.17-rc5' of git://git.kernel...
git tree: upstream
final oops: https://syzkaller.appspot.com/x/report.txt?x=1563c962580000
console output: https://syzkaller.appspot.com/x/log.txt?x=1163c962580000

kernel config: https://syzkaller.appspot.com/x/.config?x=bd9738e00c1bbfb4
dashboard link: https://syzkaller.appspot.com/bug?extid=55c84106264e92ea9ada

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1788c962580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12b0887c580000

Reported-by: syzbot+55c841...@syzkaller.appspotmail.com
Fixes: f11ca2ab18e3 ("bcachefs: Guard against backpointers to unknown btrees")

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

[syzbot]

For archival purposes, forwarding an incoming command email to
syzkall...@googlegroups.com.

***

Subject: Re: [PATCH] bcachefs:fix use-after-free in __bch2_bkey_fsck_err
Author: ssran...@ee.vjti.ac.in

#syz test:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

On Sun, Oct 5, 2025 at 9:35 PM <ssran...@ee.vjti.ac.in> wrote:

> From: Shaurya Rane <ssran...@ee.vjti.ac.in>
>
> When mounting a corrupted bcachefs filesystem, KASAN detects a
> use-after-free in the error reporting path. This occurs due to a race
> between error reporting and key deletion during fsck.
>
> The sequence of events:
>
> 1. extent_ptr_validate() detects an invalid extent pointer with a
> corrupt disk offset
> 2. It calls __bch2_bkey_fsck_err() to report the error
> 3. The fsck logic decides to delete the corrupt key
> 4. Meanwhile, __bch2_bkey_fsck_err() calls bch2_bkey_val_to_text()
> 5. This traverses the extent entries via bch2_extent_ptr_to_text()
> 6. sector_to_bucket_and_offset() is called with the corrupt offset
> 7. Memory access occurs on data being concurrently freed
>
> The crash happens in bch2_extent_ptr_to_text() when it attempts to
> validate bucket boundaries using already-freed extent pointer data.
>
> Replace bch2_bkey_val_to_text() with bch2_bpos_to_text() to print
> only the key position (inode, offset, snapshot) instead of traversing
> potentially corrupt or freed extent metadata. The position is part of
> the key header and remains safe to read even as the extent data is
> being freed. The specific validation error is still printed in the
> subsequent lines, providing sufficient context for debugging.
>
> Tested successfully locally using syzbot provided reproducer
>
> Reported-by: syzbot+564efb...@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?id=564efbe31172fe908429
> Fixes: d97de0d017cd ("bcachefs: Make bkey_fsck_err() a wrapper around
> fsck_err()")
> Signed-off-by: Shaurya Rane <ssran...@ee.vjti.ac.in>
> ---
> fs/bcachefs/error.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/bcachefs/error.c b/fs/bcachefs/error.c
> index 267e73d9d7e6..b0bf08915aa2 100644
> --- a/fs/bcachefs/error.c
> +++ b/fs/bcachefs/error.c
> @@ -688,7 +688,7 @@ int __bch2_bkey_fsck_err(struct bch_fs *c,
> bch2_btree_id_to_text(&buf, from.btree);
> prt_printf(&buf, " level=%u: ", from.level);
>
> - bch2_bkey_val_to_text(&buf, c, k);
> + bch2_bpos_to_text(&buf, k.k->p);
> prt_newline(&buf);
>
> va_list args;
> --
> 2.34.1
>
>

[syzbot]

[syzbot]

syzbot suspects this issue was fixed by commit:

commit f2c61db29f277b9c80de92102fc532cc247495cd
Author: Linus Torvalds <torv...@linux-foundation.org>
Date: Mon Sep 29 20:43:52 2025 +0000

Remove bcachefs core code

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=100837cd980000
start commit: d69eb204c255 Merge tag 'net-6.17-rc5' of git://git.kernel...
git tree: upstream

kernel config: https://syzkaller.appspot.com/x/.config?x=bd9738e00c1bbfb4
dashboard link: https://syzkaller.appspot.com/bug?extid=55c84106264e92ea9ada

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1788c962580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12b0887c580000

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

#syz fix: Remove bcachefs core code