From f3c8cd8a63683f53a4e0247ef2b3cdc5132e97fa Mon Sep 17 00:00:00 2001 From: DaeMyung Kang Date: Sat, 2 May 2026 09:48:52 +0900 Subject: [PATCH] ntfs: fix copy length in ntfs_bdev_write() for non-page-aligned start 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 Reviewed-by: Hyunchul Lee Signed-off-by: Namjae Jeon --- fs/ntfs/bdev-io.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/fs/ntfs/bdev-io.c b/fs/ntfs/bdev-io.c index 67e65c88d681..27d7c2767a33 100644 --- a/fs/ntfs/bdev-io.c +++ b/fs/ntfs/bdev-io.c @@ -97,6 +97,8 @@ int ntfs_bdev_write(struct super_block *sb, void *buf, loff_t start, size_t size idx_end++; for (; idx < idx_end; idx++, from = 0) { + u32 len; + folio = read_mapping_folio(sb->s_bdev->bd_mapping, idx, NULL); if (IS_ERR(folio)) { ntfs_error(sb, "Unable to read %ld page", idx); @@ -105,9 +107,10 @@ int ntfs_bdev_write(struct super_block *sb, void *buf, loff_t start, size_t size offset = (loff_t)idx << PAGE_SHIFT; to = min_t(u32, end - offset, PAGE_SIZE); + len = to - from; - memcpy_to_folio(folio, from, buf + buf_off, to); - buf_off += to; + memcpy_to_folio(folio, from, buf + buf_off, len); + buf_off += len; folio_mark_uptodate(folio); folio_mark_dirty(folio); folio_put(folio);