sctp: 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: bbd0d59809f9 ("[SCTP]: Implement the receive and verification of AUTH chunk")
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: [e-mail]
Signed-off-by: Eric Biggers <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sctp/sm_make_chunk.c

···
 #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>
···
 		}
 	}
 
-	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

···
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <crypto/utils.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/ip.h>
···
 				 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;
 	}