kek_unwrap_key(): Fix incorrect check of unwrapped key size

Fixes CVE-2025-9230

The check is off by 8 bytes so it is possible to overread by
up to 8 bytes and overwrite up to 4 bytes.

Reviewed-by: Neil Horman <[email protected]>
Reviewed-by: Matt Caswell <[email protected]>
Reviewed-by: Tomas Mraz <[email protected]>

Author: Viktor Dukhovni

crypto/cms/cms_pwri.c

@@ -237,7 +237,7 @@ static int kek_unwrap_key(unsigned char *out, size_t *outlen,
         /* Check byte failure */
         goto err;
     }
-    if (inlen < (size_t)(tmp[0] - 4)) {
+    if (inlen < 4 + (size_t)tmp[0]) {
         /* Invalid length value */
         goto err;
     }