[syzbot] [exfat?] KASAN: stack-out-of-bounds Read in exfat_nls_to_utf16

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 6093a688a07d Merge tag 'char-misc-6.18-rc1' of git://git.k..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1149dee2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=e2b03b8b7809165e
dashboard link: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
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=12451a7c580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=11aa7942580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/15a8a043eb77/disk-6093a688.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/2e5dfeaf5d0e/vmlinux-6093a688.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3d11ec676d6d/bzImage-6093a688.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/0204bf2eab84/mount_0.gz
fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=142895cd980000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+98cc76...@syzkaller.appspotmail.com

exFAT-fs (loop0): failed to load upcase table (idx : 0x00010000, chksum : 0xe3865569, utbl_chksum : 0xe619d30d)
==================================================================
BUG: KASAN: stack-out-of-bounds in exfat_nls_to_ucs2 fs/exfat/nls.c:619 [inline]
BUG: KASAN: stack-out-of-bounds in exfat_nls_to_utf16+0xac8/0xc10 fs/exfat/nls.c:647
Read of size 1 at addr ff[ 114.258372][ T5975] Read of size 1 at addr ffffc90003c37710 by task syz.0.17/5975

CPU: 1 UID: 0 PID: 5975 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
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
exfat_nls_to_ucs2 fs/exfat/nls.c:619 [inline]
exfat_nls_to_utf16+0xac8/0xc10 fs/exfat/nls.c:647
exfat_ioctl_set_volume_label fs/exfat/file.c:524 [inline]
exfat_ioctl+0x818/0x1100 fs/exfat/file.c:554
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
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: 0033:0x7fed7375eec9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 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:00007fff5eae5808 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fed739b5fa0 RCX: 00007fed7375eec9
RDX: 00002000000007c0 RSI: 0000000041009432 RDI: 0000000000000004
RBP: 00007fed737e1f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fed739b5fa0 R14: 00007fed739b5fa0 R15: 0000000000000003
</TASK>

The buggy address belongs to stack of task syz.0.17/5975
and is located at offset 304 in frame:
exfat_ioctl+0x0/0x1100 fs/exfat/file.c:33

This frame has 7 objects:
[32, 36) 'lossy.i'
[48, 304) 'label.i50'
[368, 888) 'uniname.i51'
[1024, 1280) 'label.i'
[1344, 1864) 'uniname.i'
[2000, 2024) 'range.i'
[2064, 2144) 'ia.i'

The buggy address belongs to a 8-page vmalloc region starting at 0xffffc90003c30000 allocated at copy_process+0x545/0x3ae0 kernel/fork.c:2012
The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff888026a18f00 pfn:0x26a18
memcg:ffff888022f68802
flags: 0x80000000000000(node=0|zone=1)
raw: 0080000000000000 0000000000000000 dead000000000122 0000000000000000
raw: ffff888026a18f00 0000000000000000 00000001ffffffff ffff888022f68802
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x2dc2(GFP_KERNEL|__GFP_HIGHMEM|__GFP_ZERO|__GFP_NOWARN), pid 3634, tgid 3634 (kworker/u8:13), ts 113475141729, free_ts 113296085908
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
alloc_frozen_pages_noprof mm/mempolicy.c:2487 [inline]
alloc_pages_noprof+0xcf/0x1e0 mm/mempolicy.c:2507
vm_area_alloc_pages mm/vmalloc.c:3647 [inline]
__vmalloc_area_node mm/vmalloc.c:3724 [inline]
__vmalloc_node_range_noprof+0x96c/0x12d0 mm/vmalloc.c:3897
__vmalloc_node_noprof+0xc2/0x110 mm/vmalloc.c:3960
alloc_thread_stack_node kernel/fork.c:311 [inline]
dup_task_struct+0x3d4/0x830 kernel/fork.c:881
copy_process+0x545/0x3ae0 kernel/fork.c:2012
kernel_clone+0x224/0x7c0 kernel/fork.c:2609
user_mode_thread+0xdd/0x140 kernel/fork.c:2685
call_usermodehelper_exec_sync kernel/umh.c:132 [inline]
call_usermodehelper_exec_work+0x9c/0x230 kernel/umh.c:163
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x436/0x7d0 arch/x86/kernel/process.c:158
page last free pid 28 tgid 28 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
free_pages_prepare mm/page_alloc.c:1394 [inline]
__free_frozen_pages+0xfb6/0x1140 mm/page_alloc.c:2906
rcu_do_batch kernel/rcu/tree.c:2605 [inline]
rcu_core kernel/rcu/tree.c:2861 [inline]
rcu_cpu_kthread+0xbf3/0x1b50 kernel/rcu/tree.c:2949
smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x436/0x7d0 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Memory state around the buggy address:
ffffc90003c37600: 04 f2 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffffc90003c37680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffffc90003c37700: 00 00 f2 f2 f2 f2 f2 f2 f2 f2 00 00 00 00 00 00
^
ffffc90003c37780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffffc90003c37800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================


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