a negative ntfs superblock mft_lcn can cause a wild memory reference

If the ntfs superblock's mft_lcn is negative, that will pass tests such as

        ll = le64_to_cpu(b->mft_lcn);
        if (ll >= vol->nr_clusters) {

and

        vol->mft_zone_end = vol->mft_lcn + mft_zone_size;
        while (vol->mft_zone_end >= vol->nr_clusters) {

because the variables are signed.

The attached corrupt NTFS image exploits this to cause index to be out of bounds in ntfs_cluster_alloc(), e.g. for

                if (vol->lcn_empty_bits_per_page[index] == 0)
                        goto next_bmp_pos;

index is derived from mft_lcn via mft_zone_end, data1_zone_pos, zone_start, and bmp_pos.

# wget http://www.rtmrtm.org/rtm/ntfs8b.img.gz
# gunzip ntfs8b.img.gz
# cp ntfs8b.img junk
# mount -t ntfs -o loop junk /mnt
# echo x > /mnt/x
ntfs: (device loop0): check_mft_mirror(): $MFT and $MFTMirr record 0 do not match.  Run chkdsk.
BUG: unable to handle page fault for address: ffff80010e38ef58
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP DEBUG_PAGEALLOC PTI
CPU: 10 UID: 0 PID: 1264 Comm: tcsh Not tainted 7.2.0-rc7-00016-g3d6d817622b0 #40 PREEMPT(full)
Hardware name: FreeBSD BHYVE/BHYVE, BIOS 14.0 10/17/2021
RIP: 0010:ntfs_cluster_alloc+0x376/0x12a0
Call Trace:
 <TASK>
 ? inode_io_list_move_locked+0xa1/0xe0
 ? folio_mark_dirty+0x33/0x60
 ntfs_mft_data_extend_allocation_nolock+0x15b/0xa30
 ntfs_mft_record_alloc+0x921/0x1790
 __ntfs_create+0x1c4/0xd40
 ? ntfs_attr_put_search_ctx+0x3b/0x50
 ? kmem_cache_free+0x207/0x2f0
 ? __mark_inode_dirty+0x16a/0x3b0
 ntfs_create+0xc1/0x170
 path_openat+0xfe0/0x12f0
 ? do_syscall_64+0xf9/0x540
 ? do_getname+0x2e/0x180
 do_file_open+0xdf/0x1a0
 do_sys_openat2+0x79/0xe0
 __x64_sys_creat+0x49/0x70
 x64_sys_call+0x1bfa/0x2130
 do_syscall_64+0xf9/0x540
 ? exc_page_fault+0x76/0x130
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Robert Morris
[email protected]

Hi Robert,

I have submitted a patch to fix this issue:

https://lore.kernel.org/all/[email protected]/

2026년 8월 18일 (화) 오전 6:03, <[email protected]>님이 작성:

Thanks,
Hyunchul