[PATCH v3 2/3] ntfs: bound the look-ahead attribute-list entry in ntfs_external_attr_find()

[Bryam Vargas <hexlabsecurity@proton.me>]

When resolving an attribute lookup with a non-zero @lowest_vcn,
ntfs_external_attr_find() peeks at the next $ATTRIBUTE_LIST entry to
decide whether to keep searching, but bounds that not-yet-validated
entry only with "(u8 *)next_al_entry + 6 < al_end" (which proves just
bytes 0..6 are in range) and "(u8 *)next_al_entry + length <= al_end"
with an attacker-controlled, non-8-aligned length. It then reads
next_al_entry->lowest_vcn (an __le64 at offset 8) and the name at
next_al_entry->name_offset, both of which can lie past al_end -- the
exact end of the kvmalloc'd attribute-list buffer (allocated at the
on-disk attr_list_size, no rounding). A crafted on-disk $ATTRIBUTE_LIST
whose last entry sits a few bytes before al_end therefore yields a slab
out-of-bounds read when the inode is read.

Validate the look-ahead entry with ntfs_attr_list_entry_is_valid() (added
in patch 1/3) before dereferencing lowest_vcn and the name, so the same
fixed-header, length and name bounds the main attribute-list walk uses now
guard this read too.

Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
---
v3 (Hyunchul Lee review): use the extracted ntfs_attr_list_entry_is_valid()
   helper (added in 1/3) instead of an open-coded bound, so the look-ahead
   gets the same fixed-header, 8-byte-aligned-length and name checks as the
   main walk.
v2: dropped the redundant Reported-by; reproducer/KASAN splat omitted on the
   public list (available to the maintainers on request).

Confirmed under KASAN on the earlier revision: a slab out-of-bounds read of
next_al_entry->lowest_vcn during inode read of a crafted image; a benign
mkntfs image is KASAN-clean (control). Arch-independent (lowest_vcn at
offset 8, fixed header offsetof(.., name) == 26; identical geometry built
-m32/-m64). Please add the appropriate Fixes: tag for the commit that
introduced ntfs_external_attr_find() in the new fs/ntfs driver (v7.1 merge
window).

 fs/ntfs/attrib.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/ntfs/attrib.c b/fs/ntfs/attrib.c
index abc0add6f0c4..e425c8d074c5 100644
--- a/fs/ntfs/attrib.c
+++ b/fs/ntfs/attrib.c
@@ -1185,9 +1185,8 @@ static int ntfs_external_attr_find(const __le32 type,
 		 * we have reached the right one or the search has failed.
 		 */
 		if (lowest_vcn && (u8 *)next_al_entry >= al_start &&
-				(u8 *)next_al_entry + 6 < al_end &&
-				(u8 *)next_al_entry + le16_to_cpu(
-					next_al_entry->length) <= al_end &&
+				ntfs_attr_list_entry_is_valid(next_al_entry,
+							      al_end) &&
 				le64_to_cpu(next_al_entry->lowest_vcn) <=
 					lowest_vcn &&
 				next_al_entry->type == al_entry->type &&
-- 
2.43.0