Re: [PATCH] ima: fix out-of-bounds read in xattr_verify()

On Wed, 2026-07-29 at 23:04 -0300, Lincoln Wallace wrote:
> The digest-length check in xattr_verify() mixes int and size_t:
>=20
> 	if (xattr_len - sizeof(xattr_value->type) - hash_start >=3D
> 			iint->ima_hash->length)
>=20
> 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 =3D=3D 1, hash_start =3D=3D=
 1)
> turns "1 - 1 - 1" into SIZE_MAX, which is trivially >=3D 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.
>=20
> 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.
>=20
> Rewrite the comparison so every operand stays a signed int and no
> implicit conversion to size_t can occur.
>=20
> 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(-)
>=20
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/i=
ma/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 >=3D
> -				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 t=
he
comment like:
             =C2=A0
		/*
                 * Use addition, not subtraction: sizeof() forces unsigned =
   =20
                 * math and a short xattr_len would wrap around, bypassing =
   =20
		 * this bounds check.    =20
                 */

> +		if (xattr_len >=3D (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
>=20
> ---
> base-commit: fc02acf6ac0ccde0c805c2daa9148683cdd01ba8
> change-id: 20260729-fix-ima-underflow-40bd249a9afa