CVE-2023-52933

In the Linux kernel, the following vulnerability has been resolved:

Squashfs: fix handling and sanity checking of xattr_ids count

A Sysbot [1] corrupted filesystem exposes two flaws in the handling and sanity checking of the xattr_ids count in the filesystem. Both of these flaws cause computation overflow due to incorrect typing.

In the corrupted filesystem the xattr_ids value is 4294967071, which stored in a signed variable becomes the negative number -225.

Flaw 1 (64-bit systems only):

The signed integer xattr_ids variable causes sign extension.

This causes variable overflow in the SQUASHFSXATTR*(A) macros. The variable is first multiplied by sizeof(struct squashfsxattrid) where the type of the sizeof operator is "unsigned long".

On a 64-bit system this is 64-bits in size, and causes the negative number to be sign extended and widened to 64-bits and then become unsigned. This produces the very large number 18446744073709548016 or 2^64 - 3600. This number when rounded up by SQUASHFSMETADATASIZE - 1 (8191 bytes) and divided by SQUASHFSMETADATASIZE overflows and produces a length of 0 (stored in len).

Flaw 2 (32-bit systems only):

On a 32-bit system the integer variable is not widened by the unsigned long type of the sizeof operator (32-bits), and the signedness of the variable has no effect due it always being treated as unsigned.

The above corrupted xattrids value of 4294967071, when multiplied overflows and produces the number 4294963696 or 2^32 - 3400. This number when rounded up by SQUASHFSMETADATASIZE - 1 (8191 bytes) and divided by SQUASHFSMETADATA_SIZE overflows again and produces a length of 0.

The effect of the 0 length computation:

In conjunction with the corrupted xattrids field, the filesystem also has a corrupted xattrtable_start value, where it matches the end of filesystem value of 850.

This causes the following sanity check code to fail because the incorrectly computed len of 0 matches the incorrect size of the table reported by the superblock (0 bytes).

len = SQUASHFS_XATTR_BLOCK_BYTES(*xattr_ids);
indexes = SQUASHFS_XATTR_BLOCKS(*xattr_ids);

/*
 * The computed size of the index table (len bytes) should exactly
 * match the table start and end points
*/
start = table_start + sizeof(*id_table);
end = msblk->bytes_used;

if (len != (end - start))
        return ERR_PTR(-EINVAL);

Changing the xattr_ids variable to be "usigned int" fixes the flaw on a 64-bit system. This relies on the fact the computation is widened by the unsigned long type of the sizeof operator.

Casting the variable to u64 in the above macro fixes this flaw on a 32-bit system.

It also means 64-bit systems do not implicitly rely on the type of the sizeof operator to widen the computation.

[1] https://lore.kernel.org/lkml/000000000000cd44f005f1a0f17f@google.com/