[PATCH] ntfs: fix u16 truncation of the update sequence array count check

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

ntfs_check_restart_page_header() bounds the on-disk system page size by
recomputing the update sequence array count from it:

	u16 ra_ofs, usa_count, usa_ofs, usa_end = 0;
	...
	usa_count = 1 + (logfile_system_page_size >> NTFS_BLOCK_SIZE_BITS);
	if (usa_count != le16_to_cpu(rp->usa_count))
		return false;

The right hand side is computed in u32 but stored in a u16, so the count
wraps once the system page size reaches 0x2000000.  An image claiming any
larger power of two passes the check by also claiming usa_count == 1.

Nothing else limits the value, so ntfs_check_and_load_restart_page() goes
on to kvzalloc() up to 2 GiB for the restart page and copy that many bytes
a page at a time.  The size is only rejected afterwards, by the
(size >> NTFS_BLOCK_SIZE_BITS) != usa_count test in post_read_mst_fixup().

Compute the count in a u32 so the existing check rejects an over-large
system page size instead of being defeated by the truncation.  This adds
no new limit: the array holds one u16 per protected sector plus the update
sequence number and must end within the first sector, which caps usa_count
at 240 and the system page size at 64 KiB.

Widen ra_ofs and usa_ofs as well, as ntfs_check_restart_area() already
does: both are loaded from __le16 on-disk fields and every comparison is
against a non-negative value, so no result changes.

Fixes: 1e9ea7e04472 ("Revert "fs: Remove NTFS classic"")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
 fs/ntfs/logfile.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ntfs/logfile.c b/fs/ntfs/logfile.c
index ce38ce87f8fc8..2f27ae7a3927f 100644
--- a/fs/ntfs/logfile.c
+++ b/fs/ntfs/logfile.c
@@ -27,7 +27,7 @@ static bool ntfs_check_restart_page_header(struct inode *vi,
 		struct restart_page_header *rp, s64 pos)
 {
 	u32 logfile_system_page_size, logfile_log_page_size;
-	u16 ra_ofs, usa_count, usa_ofs, usa_end = 0;
+	u32 ra_ofs, usa_count, usa_ofs, usa_end = 0;
 	bool have_usa = true;
 
 	ntfs_debug("Entering.");

base-commit: 19dc08f1f2c5563ba4da6afd5ecace8eba163aef
-- 
2.43.0