This is not a normal data I/O hot path. The single in-tree caller is the $LogFile emptying path used during read-write mount/remount, and the bug only becomes visible on NTFS volumes whose cluster_size is strictly smaller than the kernel's PAGE_SIZE (typically 4 KiB on x86_64). Per Microsoft's format command documentation, NTFS supports allocation unit sizes starting at 512 bytes, so 512 B, 1 KiB and 2 KiB clusters are uncommon but valid on-disk configurations. When cluster_size >= PAGE_SIZE every "start" passed in is page-aligned and the buggy "from != 0" path is never taken. ntfs_bdev_write() splits the write across one or more block-device folios. Inside the loop, "to" is computed as the *end byte offset* within the current page (0..PAGE_SIZE), and "from" is the start byte offset within the page (reset to 0 from the second iteration onward). The copy length should therefore be "to - from", but the current code uses "to" directly: to = min_t(u32, end - offset, PAGE_SIZE); memcpy_to_folio(folio, from, buf + buf_off, to); buf_off += to; When "from != 0" (i.e. "start" is not page-aligned) memcpy_to_folio() copies "from" extra bytes: - it reads "from" bytes past the source buffer into kernel heap; - it writes "from" bytes past the requested range into the next part of the block-device page (or, if "from + to > PAGE_SIZE", past the folio boundary entirely, which trips the VM_BUG_ON inside memcpy_to_folio() on CONFIG_DEBUG_VM=y kernels). "buf_off" is then advanced by the wrong amount, so every subsequent iteration also reads the source buffer at the wrong offset and writes the wrong content to disk. ntfs_empty_logfile() calls ntfs_bdev_write(sb, empty_buf, NTFS_CLU_TO_B(vol, lcn), vol->cluster_size); with empty_buf sized to vol->cluster_size. On a sub-PAGE_SIZE-cluster volume, any $LogFile run whose LCN is not aligned to PAGE_SIZE / cluster_size reaches the non-page-aligned path. The over-copy can read beyond empty_buf and overwrite the sectors following the requested cluster in the block-device page with unrelated kernel heap contents while $LogFile is being emptied. A userspace reducer of the same arithmetic and copy loop confirms the bug under AddressSanitizer: ASan reports a heap-buffer-overflow read past the source buffer for the buggy length, and the fixed version is ASan-clean. Compute the copy length as "to - from" and advance buf_off by the same amount. Fixes: 5218cd102aec ("ntfs: update misc operations") Link: https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/format Signed-off-by: DaeMyung Kang <charsyam@gmail.com> Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com> Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>