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 SQUASHFS_XATTR_*(A) macros. The variable is first multiplied by sizeof(struct squashfs_xattr_id) 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 SQUASHFS_METADATA_SIZE - 1 (8191 bytes) and divided by SQUASHFS_METADATA_SIZE 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 xattr_ids value of 4294967071, when multiplied overflows and produces the number 4294963696 or 2^32 - 3400. This number when rounded up by SQUASHFS_METADATA_SIZE - 1 (8191 bytes) and divided by SQUASHFS_METADATA_SIZE overflows again and produces a length of 0.

The effect of the 0 length computation:

In conjunction with the corrupted xattr_ids field, the filesystem also has a corrupted xattr_table_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/

Weakness

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• If possible, choose a language or compiler that performs automatic bounds checking.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482].
• Use libraries or frameworks that make it easier to handle numbers without unexpected consequences.
• Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++). [REF-106]
• Perform input validation on any numeric input by ensuring that it is within the expected range. Enforce that the input meets both the minimum and maximum requirements for the expected range.
• Use unsigned integers where possible. This makes it easier to perform validation for integer overflows. When signed integers are required, ensure that the range check includes minimum values as well as maximum values.
• Understand the programming language’s underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, “not-a-number” calculations, and how the language handles numbers that are too large or too small for its underlying representation. [REF-7]
• Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric representation.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/92.html

References

• https://git.kernel.org/stable/c/1369322c1de52c7b9b988b95c9903110a4566778
• https://git.kernel.org/stable/c/5c4d4a83bf1a862d80c1efff1c6e3ce33b501e2e
• https://git.kernel.org/stable/c/7fe583c9bec10cd4b76231c51b37f3e4ca646e01
• https://git.kernel.org/stable/c/997bed0f3cde78a3e639d624985bf4a95cf767e6
• https://git.kernel.org/stable/c/a7da7d01ac5ce9b369a1ac70e1197999cc6c9686
• https://git.kernel.org/stable/c/b38c3e9e0adc01956cc3e5a52e4d3f92f79d88e2
• https://git.kernel.org/stable/c/f65c4bbbd682b0877b669828b4e033b8d5d0a2dc