[PATCH] ntfs: fix copy length in ntfs_bdev_write() for non-page-aligned start

[DaeMyung Kang <charsyam@gmail.com>]

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>
---
 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);
-- 
2.43.0