[PATCH 1/2] squashfs: fix fragment index table sizing overflow on 32-bit

[Karl Mehltretter]

SQUASHFS_FRAGMENT_BYTES() multiplies the on-disk fragment count (an
unsigned int) by sizeof(struct squashfs_fragment_entry), a size_t. On
a 32-bit kernel that product is 32-bit and can wrap.

squashfs_read_fragment_index_table() sizes the fragment index table
from it, but squashfs_frag_lookup() bounds the fragment number against
msblk->fragments, the unwrapped superblock value. The two disagree: an
image declaring 0x10000001 fragments wraps the product to 16, so a
single index entry is allocated, yet the lookup still accepts fragment
0x0fffffff:

	if (fragment >= msblk->fragments)
		return -EIO;
	block = SQUASHFS_FRAGMENT_INDEX(fragment);
	...
	start_block = le64_to_cpu(msblk->fragment_index[block]);

block is then 524287 and the read lands ~4MB past an 8-byte
allocation. On a 32-bit build KASAN catches it when the crafted image
is mounted and the file is stat'd.

Cast to u64 in the macro so the multiplication is 64-bit on all
targets. After conversion to index-table entries,
SQUASHFS_FRAGMENT_INDEX_BYTES() is at most 64 MiB for any u32 count,
so it fits both the unsigned int local and the int argument it feeds.
64-bit builds are unchanged.

Fixes: ffae2cd73a9e ("Squashfs: header files")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-5
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
 fs/squashfs/squashfs_fs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/squashfs/squashfs_fs.h b/fs/squashfs/squashfs_fs.h
index a955d9369749f..93436c7d80c97 100644
--- a/fs/squashfs/squashfs_fs.h
+++ b/fs/squashfs/squashfs_fs.h
@@ -136,7 +136,7 @@ static inline int squashfs_block_size(__le32 raw)
 
 /* fragment and fragment table defines */
 #define SQUASHFS_FRAGMENT_BYTES(A)	\
-				((A) * sizeof(struct squashfs_fragment_entry))
+			((u64)(A) * sizeof(struct squashfs_fragment_entry))
 
 #define SQUASHFS_FRAGMENT_INDEX(A)	(SQUASHFS_FRAGMENT_BYTES(A) / \
 					SQUASHFS_METADATA_SIZE)
-- 
2.53.0