On Wed, 2026-07-29 at 23:04 -0300, Lincoln Wallace wrote:
> The digest-length check in xattr_verify() mixes int and size_t:
>
> if (xattr_len - sizeof(xattr_value->type) - hash_start >=
> iint->ima_hash->length)
>
> sizeof() yields size_t, so the usual arithmetic conversions promote
> the whole left-hand side to unsigned 64-bit before the subtraction
> runs. For a truncated xattr this underflows instead of going negative:
> a 1-byte IMA_XATTR_DIGEST_NG xattr (xattr_len == 1, hash_start == 1)
> turns "1 - 1 - 1" into SIZE_MAX, which is trivially >= ima_hash->length.
> The check then passes and the following memcmp() reads
> iint->ima_hash->length bytes starting past the end of the buffer
> vfs_getxattr_alloc() allocated for it.
>
> Nothing upstream clamps xattr_len back into a safe range first:
> ima_get_hash_algo() only special-cases xattr_len < 2 to pick a default
> algorithm, and evm_verifyxattr() returns INTEGRITY_UNKNOWN rather than
> failing when no HMAC key is loaded, so a truncated security.ima value
> reaches the length check as-is.
>
> Rewrite the comparison so every operand stays a signed int and no
> implicit conversion to size_t can occur.
>
> Fixes: 3ea7a56067e6 ("ima: provide hash algo info in the xattr")
> Cc: [email protected]
> Signed-off-by: Lincoln Wallace <[email protected]>
> ---
> security/integrity/ima/ima_appraise.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/security/integrity/ima/ima_appraise.c
> b/security/integrity/ima/ima_appraise.c
> index 18d0d9154317..e39627f9c46c 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -274,8 +274,12 @@ static int xattr_verify(enum ima_hooks func, struct
> ima_iint_cache *iint,
> } else {
> set_bit(IMA_DIGSIG, &iint->atomic_flags);
> }
> - if (xattr_len - sizeof(xattr_value->type) - hash_start >=
> - iint->ima_hash->length)
> + /*
> + * Keep every operand int: sizeof() is size_t and would hide
> + * a signed underflow as SIZE_MAX. Do not rewrite as
> subtraction.
> + */
Thanks, Lincoln.
Just using addition would solve the underflow issue. How about rewriting the
comment like:
/*
* Use addition, not subtraction: sizeof() forces unsigned
* math and a short xattr_len would wrap around, bypassing
* this bounds check.
*/
> + if (xattr_len >= (int)sizeof(xattr_value->type) + hash_start +
> + (int)iint->ima_hash->length)
Keeping the "(int)" here is not required, but clearer.
Thanks,
Mimi
> /*
> * xattr length may be longer. md5 hash in previous
> * version occupied 20 bytes in xattr, instead of 16
>
> ---
> base-commit: fc02acf6ac0ccde0c805c2daa9148683cdd01ba8
> change-id: 20260729-fix-ima-underflow-40bd249a9afa
