ipv6: sr: Fix MAC comparison to be constant-time

JIRA: https://issues.redhat.com/browse/RHEL-115599

commit a458b29
Author: Eric Biggers <ebiggers@kernel.org>
Date:   Mon Aug 18 13:27:24 2025 -0700

    ipv6: sr: Fix MAC comparison to be constant-time

    To prevent timing attacks, MACs need to be compared in constant time.
    Use the appropriate helper function for this.

    Fixes: bf355b8 ("ipv6: sr: add core files for SR HMAC support")
    Cc: stable@vger.kernel.org
    Signed-off-by: Eric Biggers <ebiggers@kernel.org>
    Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
    Link: https://patch.msgid.link/20250818202724.15713-1-ebiggers@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

net/ipv6/seg6_hmac.c

@@ -35,6 +35,7 @@
 #include <net/xfrm.h>
 
 #include <crypto/hash.h>
+#include <crypto/utils.h>
 #include <net/seg6.h>
 #include <net/genetlink.h>
 #include <net/seg6_hmac.h>
@@ -271,7 +272,7 @@ bool seg6_hmac_validate_skb(struct sk_buff *skb)
 	if (seg6_hmac_compute(hinfo, srh, &ipv6_hdr(skb)->saddr, hmac_output))
 		return false;
 
-	if (memcmp(hmac_output, tlv->hmac, SEG6_HMAC_FIELD_LEN) != 0)
+	if (crypto_memneq(hmac_output, tlv->hmac, SEG6_HMAC_FIELD_LEN))
 		return false;
 
 	return true;