crypto: pkcs7 - Use constant-time message digest comparison

The message digest comparison in pkcs7_verify() uses memcmp(), whose execution time depends on the position of the first mismatched byte. This creates a timing side-channel that could allow an attacker to forge PKCS#7 signatures byte by byte.

Replace memcmp() with crypto_memneq() to perform a constant-time comparison, consistent with the rest of the crypto subsystem (gcm, ccm, chacha20poly1305, rsassa-pkcs1, krb5, verify_pefile, etc.). Add an explicit <crypto/utils.h> include since this file did not previously pull in the crypto_memneq() declaration.

---
 crypto/asymmetric_keys/pkcs7_verify.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

index 474e2c1ae21b..8c619b252ca5 100644
@@ -14,6 +14,7 @@
 #include <crypto/hash.h>
 #include <crypto/hash_info.h>
 #include <crypto/public_key.h>
+#include <crypto/utils.h>
 #include "pkcs7_parser.h"
 
 /*

@@ -93,8 +94,8 @@ static int pkcs7_digest(struct pkcs7_message *pkcs7,
 			goto error;
 		}
 
-		if (memcmp(sig->m, sinfo->msgdigest,
-			   sinfo->msgdigest_len) != 0) {
+		if (crypto_memneq(sig->m, sinfo->msgdigest,
+				  sinfo->msgdigest_len)) {
 			pr_warn("Sig %u: Message digest doesn't match\n",
 				sinfo->index);
 			ret = -EKEYREJECTED;

2.25.1