tls: rx: use a define for tag length

TLS 1.3 has to strip padding, and it starts out 16 bytes
from the end of the record. Make it clear this is because
of the auth tag.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tls.h

@@ -64,6 +64,7 @@
 #define TLS_AAD_SPACE_SIZE		13
 
 #define MAX_IV_SIZE			16
+#define TLS_TAG_SIZE			16
 #define TLS_MAX_REC_SEQ_SIZE		8
 
 /* For CCM mode, the full 16-bytes of IV is made of '4' fields of given sizes.

net/tls/tls_sw.c

@@ -136,9 +136,9 @@ static int padding_length(struct tls_prot_info *prot, struct sk_buff *skb)
 
 	/* Determine zero-padding length */
 	if (prot->version == TLS_1_3_VERSION) {
+		int back = TLS_TAG_SIZE + 1;
 		char content_type = 0;
 		int err;
-		int back = 17;
 
 		while (content_type == 0) {
 			if (back > rxm->full_len - prot->prepend_size)
@@ -2496,7 +2496,7 @@ int tls_set_sw_offload(struct sock *sk, struct tls_context *ctx, int tx)
 
 	/* Sanity-check the sizes for stack allocations. */
 	if (iv_size > MAX_IV_SIZE || nonce_size > MAX_IV_SIZE ||
-	    rec_seq_size > TLS_MAX_REC_SEQ_SIZE) {
+	    rec_seq_size > TLS_MAX_REC_SEQ_SIZE || tag_size != TLS_TAG_SIZE) {
 		rc = -EINVAL;
 		goto free_priv;
 	}