sctp: Fix MAC comparison to be constant-time

commit dd91c79 upstream.

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

Fixes: bbd0d59 ("[SCTP]: Implement the receive and verification of AUTH chunk")
Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20250818205426.30222-3-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Eric Biggers

net/sctp/sm_make_chunk.c

@@ -31,6 +31,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <crypto/hash.h>
+#include <crypto/utils.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/ip.h>
@@ -1796,7 +1797,7 @@ struct sctp_association *sctp_unpack_cookie(
 		}
 	}
 
-	if (memcmp(digest, cookie->signature, SCTP_SIGNATURE_SIZE)) {
+	if (crypto_memneq(digest, cookie->signature, SCTP_SIGNATURE_SIZE)) {
 		*error = -SCTP_IERROR_BAD_SIG;
 		goto fail;
 	}

net/sctp/sm_statefuns.c

@@ -30,6 +30,7 @@
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <crypto/utils.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/ip.h>
@@ -4417,7 +4418,7 @@ static enum sctp_ierror sctp_sf_authenticate(
 				 sh_key, GFP_ATOMIC);
 
 	/* Discard the packet if the digests do not match */
-	if (memcmp(save_digest, digest, sig_len)) {
+	if (crypto_memneq(save_digest, digest, sig_len)) {
 		kfree(save_digest);
 		return SCTP_IERROR_BAD_SIG;
 	}