[syzbot] [bcachefs?] KASAN: use-after-free Read in bch2_prt_printf

[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=10d67162580000
kernel config: https://syzkaller.appspot.com/x/.config?x=385d836f07b5a70d
dashboard link: https://syzkaller.appspot.com/bug?extid=7f176adb30b21606c5fc
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=14d67162580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1412c962580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-d69eb204.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e24d9eda2154/vmlinux-d69eb204.xz
kernel image: https://storage.googleapis.com/syzbot-assets/213a30ed1023/bzImage-d69eb204.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/aa6322da89ec/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7f176a...@syzkaller.appspotmail.com

bcachefs (loop0): Doing compatible version upgrade from 1.13: inode_has_child_snapshots to 1.28: inode_has_case_insensitive
running recovery passes: check_allocations,check_extents_to_backpointers,check_inodes
==================================================================
BUG: KASAN: use-after-free in string_nocheck lib/vsprintf.c:653 [inline]
BUG: KASAN: use-after-free in string+0x231/0x2b0 lib/vsprintf.c:735
Read of size 1 at addr ffff888050fe74f7 by task syz.0.17/5533

CPU: 0 UID: 0 PID: 5533 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xca/0x240 mm/kasan/report.c:482
kasan_report+0x118/0x150 mm/kasan/report.c:595
string_nocheck lib/vsprintf.c:653 [inline]
string+0x231/0x2b0 lib/vsprintf.c:735
vsnprintf+0x739/0xf00 lib/vsprintf.c:2926
bch2_prt_printf+0x1c6/0x740 fs/bcachefs/printbuf.c:183
bch2_dirent_to_text+0x2bd/0xd20 fs/bcachefs/dirent.c:220
__bch2_bkey_fsck_err+0x3f7/0x540 fs/bcachefs/error.c:691
bch2_dirent_validate+0x7b3/0xfb0 fs/bcachefs/dirent.c:164
bch2_bkey_val_validate+0x202/0x3e0 fs/bcachefs/bkey_methods.c:143
btree_node_bkey_val_validate fs/bcachefs/btree_io.c:880 [inline]
bch2_btree_node_read_done+0x4051/0x5550 fs/bcachefs/btree_io.c:1303
btree_node_read_work+0x40e/0xe60 fs/bcachefs/btree_io.c:1440
bch2_btree_node_read+0x887/0x2a00 fs/bcachefs/btree_io.c:-1
__bch2_btree_root_read fs/bcachefs/btree_io.c:1906 [inline]
bch2_btree_root_read+0x5f0/0x760 fs/bcachefs/btree_io.c:1928
read_btree_roots+0x2c6/0x840 fs/bcachefs/recovery.c:615
bch2_fs_recovery+0x261f/0x3a50 fs/bcachefs/recovery.c:1006
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: 0033:0x7fd00ed9038a
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:00007fff65054f18 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007fff65054fa0 RCX: 00007fd00ed9038a
RDX: 0000200000000040 RSI: 0000200000004940 RDI: 00007fff65054f60
RBP: 0000200000000040 R08: 00007fff65054fa0 R09: 0000000000004000
R10: 0000000000004000 R11: 0000000000000246 R12: 0000200000004940
R13: 00007fff65054f60 R14: 000000000000496e R15: 0000200000000000
</TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff888050fe7dc0 pfn:0x50fe7
flags: 0x4fff00000000000(node=1|zone=1|lastcpupid=0x7ff)
page_type: f0(buddy)
raw: 04fff00000000000 ffffea00013e8dc8 ffffea000139e008 0000000000000000
raw: ffff888050fe7dc0 0000000000000000 00000000f0000000 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x440dc0(GFP_KERNEL_ACCOUNT|__GFP_ZERO|__GFP_COMP), pid 5454, tgid 5454 (dhcpcd-run-hook), ts 104016831092, free_ts 104102545739
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
prep_new_page mm/page_alloc.c:1859 [inline]
get_page_from_freelist+0x21e4/0x22c0 mm/page_alloc.c:3858
__alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5148
alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2416
alloc_frozen_pages_noprof mm/mempolicy.c:2487 [inline]
alloc_pages_noprof+0xa9/0x190 mm/mempolicy.c:2507
pagetable_alloc_noprof include/linux/mm.h:2881 [inline]
__pte_alloc_one_noprof include/asm-generic/pgalloc.h:75 [inline]
pte_alloc_one+0x21/0x170 arch/x86/mm/pgtable.c:18
__pte_alloc+0x25/0x1a0 mm/memory.c:452
copy_pte_range mm/memory.c:1107 [inline]
copy_pmd_range+0x6a91/0x71d0 mm/memory.c:1261
copy_pud_range mm/memory.c:1298 [inline]
copy_p4d_range mm/memory.c:1322 [inline]
copy_page_range+0xc14/0x1270 mm/memory.c:1410
dup_mmap+0xf57/0x1ac0 mm/mmap.c:1834
dup_mm kernel/fork.c:1485 [inline]
copy_mm+0x13c/0x4b0 kernel/fork.c:1537
copy_process+0x1706/0x3c00 kernel/fork.c:2175
kernel_clone+0x21e/0x840 kernel/fork.c:2605
__do_sys_clone kernel/fork.c:2748 [inline]
__se_sys_clone kernel/fork.c:2732 [inline]
__x64_sys_clone+0x18b/0x1e0 kernel/fork.c:2732
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
page last free pid 15 tgid 15 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1395 [inline]
__free_frozen_pages+0xbc4/0xd30 mm/page_alloc.c:2895
pagetable_free include/linux/mm.h:2898 [inline]
pagetable_dtor_free include/linux/mm.h:2996 [inline]
__tlb_remove_table+0x2d2/0x3b0 include/asm-generic/tlb.h:220
__tlb_remove_table_free mm/mmu_gather.c:227 [inline]
tlb_remove_table_rcu+0x85/0x100 mm/mmu_gather.c:290
rcu_do_batch kernel/rcu/tree.c:2605 [inline]
rcu_core+0xcab/0x1770 kernel/rcu/tree.c:2861
handle_softirqs+0x283/0x870 kernel/softirq.c:579
run_ksoftirqd+0x9b/0x100 kernel/softirq.c:968
smpboot_thread_fn+0x53f/0xa60 kernel/smpboot.c:160
kthread+0x70e/0x8a0 kernel/kthread.c:463
ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Memory state around the buggy address:
ffff888050fe7380: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888050fe7400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888050fe7480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
^
ffff888050fe7500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888050fe7580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


---
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[syzbot]

syzbot has bisected this issue to:

commit c21f41f6905be4fc5059a10a5bba94105ba87269
Author: Kent Overstreet <kent.ov...@linux.dev>
Date: Thu Apr 24 21:55:20 2025 +0000

bcachefs: bch2_dirent_to_text() shows casefolded dirents

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=12c65312580000
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=11c65312580000
console output: https://syzkaller.appspot.com/x/log.txt?x=16c65312580000
kernel config: https://syzkaller.appspot.com/x/.config?x=bd9738e00c1bbfb4
dashboard link: https://syzkaller.appspot.com/bug?extid=7f176adb30b21606c5fc
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=178ec962580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10097162580000

Reported-by: syzbot+7f176a...@syzkaller.appspotmail.com
Fixes: c21f41f6905b ("bcachefs: bch2_dirent_to_text() shows casefolded dirents")

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