[PATCH] ntfs: widen the sub-block offsets in ntfs_decompress()

[Zhan Xusheng]

ntfs_decompress() tracks where the current sub-block starts and ends
inside the destination page with two u16s:

	u16 do_sb_start;
	u16 do_sb_end;
	...
	do_sb_start = *dest_ofs;
	do_sb_end = do_sb_start + NTFS_SB_SIZE;

*dest_ofs is an int page offset masked with ~PAGE_MASK, and
NTFS_SB_SIZE is 0x1000, so do_sb_end needs PAGE_SIZE + 0xfff of range.
That exceeds u16 once PAGE_SIZE reaches 0x10000: the last sub-block of a
page starts at 0xf000 and do_sb_end wraps from 0x10000 to 0.

The zero-fill that pads a short sub-block then computes a negative
length:

	if (dp_addr < dp_sb_end) {
		int nr_bytes = do_sb_end - *dest_ofs;

		memset(dp_addr, 0, nr_bytes);

A sub-block made only of symbol tokens reaches this without consulting
do_sb_end on the way, because that path just does

	*dp_addr++ = *cb++;
	++*dest_ofs;

and is bounded by dp_addr against dp_sb_end.  With do_sb_start at 0xf000
and eight literal bytes emitted, nr_bytes is -61448 and the memset()
runs for 18446744073709490168 bytes.

Getting there is cheap: fifteen sub-blocks of eleven bytes each advance
*dest_ofs to 0xf000, since the same zero-fill pads each of them out to
0x1000 while do_sb_end is still exact, and the sixteenth one triggers
the wrap.  A compression block is 0x10000 bytes with 4 KiB clusters and
the fixed compression unit of 4, so all sixteen fit with room to spare.

Phrase tokens are bounded against do_sb_end and so return -EOVERFLOW
instead, and the

	if (*dest_index == dest_max_index && do_sb_end > dest_max_ofs)

check at the top of the sub-block stops firing for the same reason.

Use int for both, matching *dest_ofs and dest_max_ofs.  The narrow type
came in with the original import and survived the removal and revert of
the driver.

Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
Fixes: 1e9ea7e04472 ("Revert "fs: Remove NTFS classic"")
---
 fs/ntfs/compress.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ntfs/compress.c b/fs/ntfs/compress.c
index 075b57fc1de61..82c7c7d290737 100644
--- a/fs/ntfs/compress.c
+++ b/fs/ntfs/compress.c
@@ -182,8 +182,8 @@ static int ntfs_decompress(struct page *dest_pages[], int completed_pages[],
 	u8 *dp_addr;		/* Current pointer into dp. */
 	u8 *dp_sb_start;	/* Start of current sub-block in dp. */
 	u8 *dp_sb_end;		/* End of current sb in dp (dp_sb_start + NTFS_SB_SIZE). */
-	u16 do_sb_start;	/* @dest_ofs when starting this sub-block. */
-	u16 do_sb_end;		/* @dest_ofs of end of this sb (do_sb_start + NTFS_SB_SIZE). */
+	int do_sb_start;	/* @dest_ofs when starting this sub-block. */
+	int do_sb_end;		/* @dest_ofs of end of this sb (do_sb_start + NTFS_SB_SIZE). */
 
 	/* Variables for tag and token parsing. */
 	u8 tag;			/* Current tag. */

base-commit: 6c377d19d4a5116d9bec5203aa3c6c11523e7898
-- 
2.43.0