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

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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.

Fixes: 1e9ea7e044 ("Revert "fs: Remove NTFS classic"")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Reviewed-by: Baolin Liu <liubaolin@kylinos.cn>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
This commit is contained in:
Zhan Xusheng 2026-10-09 15:55:23 +08:00 committed by Namjae Jeon
parent b08619bf3c
commit d45f99f816
1 changed files with 2 additions and 2 deletions
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4
fs/ntfs/compress.c
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@ -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. */