[PATCH v5] fs/ntfs3: reject out-of-range evcn in mi_enum_attr()

In mi_enum_attr(), the start/end VCN validation for non-resident
attributes is:

	if (svcn > evcn + 1) goto out;

When evcn is U64_MAX the "evcn + 1" expression wraps to 0 and any svcn
passes the check. For evcn values close to U64_MAX (but not equal to it)
the right-hand side is still a meaningless near-wrap upper bound, so a
malformed on-disk attribute with svcn == 0 and evcn near U64_MAX can pass
mi_enum_attr() unrejected.

VCN (virtual cluster number) is a cluster index, so any valid evcn is
bounded by the volume's total cluster count, which ntfs3 holds in
sbi->used.bitmap.nbits (set up in ntfs_init_from_boot() before any caller
of mi_enum_attr() runs). Reject evcn values that fall outside this range.

However, an empty non-resident attribute (no allocated clusters) is
legitimately encoded with svcn == 0 and evcn == -1 (U64_MAX), e.g. via
attr->nres.evcn = cpu_to_le64((u64)vcn - 1) with vcn == 0. That sentinel
must keep passing, so exclude evcn == U64_MAX from the range check. The
existing "svcn > evcn + 1" test still tolerates the sentinel ("0 > 0" is
false) and continues to require svcn == 0 for it, while the range check
rejects every other out-of-range evcn and thereby also defuses the
"evcn + 1" wraparound.

svcn does not need its own bound: once evcn < nbits, "svcn > evcn + 1"
implies svcn <= nbits.

Fixes: 013ff63b6494 ("fs/ntfs3: Add more attributes checks in mi_enum_attr()")
Signed-off-by: Zhan Xusheng <[email protected]>
---
 fs/ntfs3/record.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/fs/ntfs3/record.c b/fs/ntfs3/record.c
index 32bdb034c2a3..d6ca4ac35a45 100644
--- a/fs/ntfs3/record.c
+++ b/fs/ntfs3/record.c
@@ -202,7 +202,7 @@ struct ATTRIB *mi_enum_attr(struct ntfs_inode *ni, struct mft_inode *mi,
 	u32 used = le32_to_cpu(rec->used);
 	u32 t32, off, asize, prev_type;
 	u16 t16;
-	u64 data_size, alloc_size, tot_size;
+	u64 svcn, evcn, data_size, alloc_size, tot_size;
 
 	if (!attr) {
 		u32 total = le32_to_cpu(rec->total);
@@ -310,8 +310,22 @@ struct ATTRIB *mi_enum_attr(struct ntfs_inode *ni, struct mft_inode *mi,
 	if (t32 && le16_to_cpu(attr->name_off) + t32 > t16)
 		goto out;
 
-	/* Check start/end vcn. */
-	if (le64_to_cpu(attr->nres.svcn) > le64_to_cpu(attr->nres.evcn) + 1)
+	/*
+	 * Check start/end vcn.  svcn == 0 with evcn == -1 (U64_MAX) is the
+	 * sentinel for an empty non-resident attribute (no allocated
+	 * clusters) and must be accepted: "svcn > evcn + 1" tolerates it,
+	 * since "(u64)-1 + 1" is 0 and "0 > 0" is false.
+	 *
+	 * For a non-empty attribute evcn is a cluster index and must lie
+	 * within the volume (sbi->used.bitmap.nbits, set up in
+	 * ntfs_init_from_boot() before any caller of mi_enum_attr() runs).
+	 * Bounding evcn also prevents a malformed value close to U64_MAX
+	 * from slipping through the near-wrap "evcn + 1" upper bound.
+	 */
+	svcn = le64_to_cpu(attr->nres.svcn);
+	evcn = le64_to_cpu(attr->nres.evcn);
+	if (svcn > evcn + 1 ||
+	    (evcn != U64_MAX && evcn >= mi->sbi->used.bitmap.nbits))
 		goto out;
 
 	data_size = le64_to_cpu(attr->nres.data_size);
-- 
2.43.0