[syzbot] [fs?] KASAN: use-after-free Read in hpfs_count_dnodes

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 72761a7e3122 Merge tag 'driver-core-6.18-rc3' of git://git..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17a293cd980000
kernel config: https://syzkaller.appspot.com/x/.config?x=8345ce4ce316ca28
dashboard link: https://syzkaller.appspot.com/bug?extid=7d1563afac6cb196a444
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=165c1e7c580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=129427e2580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/733feb854771/disk-72761a7e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/94891a491db1/vmlinux-72761a7e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/1dc1e26ee843/bzImage-72761a7e.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/2990601c297c/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7d1563...@syzkaller.appspotmail.com

HPFS: dnode_end_de: dnode->first_free = 7b3184b6
HPFS: de_next_de: de->length = 0
HPFS: dnode_end_de: dnode->first_free = 7b3184b6
HPFS: de_next_de: de->length = 0
HPFS: dnode_end_de: dnode->first_free = 7b3184b6
HPFS: de_next_de: de->length = 0
HPFS: dnode_end_de: dnode->first_free = 7b3184b6
==================================================================
BUG: KASAN: use-after-free in hpfs_count_dnodes+0x854/0xb20 fs/hpfs/dnode.c:773
Read of size 2 at addr ffff8880471a64d0 by task syz.0.17/5986

CPU: 1 UID: 0 PID: 5986 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
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
hpfs_count_dnodes+0x854/0xb20 fs/hpfs/dnode.c:773
hpfs_read_inode+0xc52/0x1010 fs/hpfs/inode.c:128
hpfs_fill_super+0x12a9/0x2050 fs/hpfs/super.c:650
get_tree_bdev_flags+0x40e/0x4d0 fs/super.c:1691
vfs_get_tree+0x92/0x2b0 fs/super.c:1751
fc_mount fs/namespace.c:1208 [inline]
do_new_mount_fc fs/namespace.c:3651 [inline]
do_new_mount+0x302/0xa10 fs/namespace.c:3727
do_mount fs/namespace.c:4050 [inline]
__do_sys_mount fs/namespace.c:4238 [inline]
__se_sys_mount+0x313/0x410 fs/namespace.c:4215
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6afb54076a
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:00007ffd5770e378 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007ffd5770e400 RCX: 00007f6afb54076a
RDX: 000020000000a000 RSI: 0000200000009ec0 RDI: 00007ffd5770e3c0
RBP: 000020000000a000 R08: 00007ffd5770e400 R09: 0000000003200041
R10: 0000000003200041 R11: 0000000000000246 R12: 0000200000009ec0
R13: 00007ffd5770e3c0 R14: 0000000000009e21 R15: 0000200000000000
</TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x5604d8ef1 pfn:0x471a6
flags: 0x80000000000000(node=0|zone=1)
raw: 0080000000000000 ffffea0000f20148 ffffea00011c6e48 0000000000000000
raw: 00000005604d8ef1 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 0, migratetype Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 5928, tgid 5928 (udevd), ts 106316997485, free_ts 115294923226
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1850
prep_new_page mm/page_alloc.c:1858 [inline]
get_page_from_freelist+0x28c0/0x2960 mm/page_alloc.c:3884
__alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5183
alloc_pages_mpol+0xd1/0x380 mm/mempolicy.c:2416
folio_alloc_mpol_noprof mm/mempolicy.c:2435 [inline]
vma_alloc_folio_noprof+0xe4/0x280 mm/mempolicy.c:2470
folio_prealloc+0x30/0x180 mm/memory.c:-1
wp_page_copy mm/memory.c:3679 [inline]
do_wp_page+0x11f4/0x4930 mm/memory.c:4140
handle_pte_fault mm/memory.c:6193 [inline]
__handle_mm_fault mm/memory.c:6318 [inline]
handle_mm_fault+0x97c/0x3400 mm/memory.c:6487
do_user_addr_fault+0xa7c/0x1380 arch/x86/mm/fault.c:1336
handle_page_fault arch/x86/mm/fault.c:1476 [inline]
exc_page_fault+0x82/0x100 arch/x86/mm/fault.c:1532
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
page last free pid 5928 tgid 5928 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1394 [inline]
free_unref_folios+0xc22/0x1860 mm/page_alloc.c:2963
folios_put_refs+0x569/0x670 mm/swap.c:1002
free_pages_and_swap_cache+0x277/0x520 mm/swap_state.c:355
__tlb_batch_free_encoded_pages mm/mmu_gather.c:136 [inline]
tlb_batch_pages_flush mm/mmu_gather.c:149 [inline]
tlb_flush_mmu_free mm/mmu_gather.c:397 [inline]
tlb_flush_mmu+0x3a0/0x680 mm/mmu_gather.c:404
tlb_finish_mmu+0xc3/0x1d0 mm/mmu_gather.c:497
exit_mmap+0x444/0xb40 mm/mmap.c:1293
__mmput+0xcb/0x3d0 kernel/fork.c:1133
exit_mm+0x1da/0x2c0 kernel/exit.c:582
do_exit+0x648/0x2300 kernel/exit.c:954
do_group_exit+0x21c/0x2d0 kernel/exit.c:1107
__do_sys_exit_group kernel/exit.c:1118 [inline]
__se_sys_exit_group kernel/exit.c:1116 [inline]
__x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1116
x64_sys_call+0x21f7/0x2200 arch/x86/include/generated/asm/syscalls_64.h:232
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
ffff8880471a6380: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff8880471a6400: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff8880471a6480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
^
ffff8880471a6500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff8880471a6580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


---
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 syzk...@googlegroups.com.

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

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

[Edward Adam Davis]

#syz test

diff --git a/fs/hpfs/map.c b/fs/hpfs/map.c
index be73233502f8..9036f1ee581d 100644
--- a/fs/hpfs/map.c
+++ b/fs/hpfs/map.c
@@ -257,7 +257,7 @@ struct dnode *hpfs_map_dnode(struct super_block *s, unsigned secno,
struct quad_buffer_head *qbh)
{
struct dnode *dnode;
- if (hpfs_sb(s)->sb_chk) {
+ /*if (hpfs_sb(s)->sb_chk)*/ {
if (hpfs_chk_sectors(s, secno, 4, "dnode")) return NULL;
if (secno & 3) {
hpfs_error(s, "dnode %08x not byte-aligned", secno);

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+7d1563...@syzkaller.appspotmail.com
Tested-by: syzbot+7d1563...@syzkaller.appspotmail.com

Tested on:

commit: fd575722 Merge tag 'sched_ext-for-6.18-rc3-fixes' of g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17fc67e2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=41ad820f608cb833

dashboard link: https://syzkaller.appspot.com/bug?extid=7d1563afac6cb196a444
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=122a1f34580000

Note: testing is done by a robot and is best-effort only.