[XFRM]: beet: fix worst case header_len calculation

esp_init_state doesn't account for the beet pseudo header in the header_len
calculation, which may result in undersized skbs hitting xfrm4_beet_output,
causing unnecessary reallocations in ip_finish_output2.

The skbs should still always have enough room to avoid causing
skb_under_panic in skb_push since we have at least 16 bytes available
from LL_RESERVED_SPACE in xfrm_state_check_space.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/esp4.c

@@ -294,7 +294,6 @@ static u32 esp4_get_mtu(struct xfrm_state *x, int mtu)
 		break;
 	case XFRM_MODE_BEET:
 		/* The worst case. */
-		mtu -= IPV4_BEET_PHMAXLEN;
 		mtu += min_t(u32, IPV4_BEET_PHMAXLEN, rem);
 		break;
 	}
@@ -409,6 +408,8 @@ static int esp_init_state(struct xfrm_state *x)
 	x->props.header_len = sizeof(struct ip_esp_hdr) + esp->conf.ivlen;
 	if (x->props.mode == XFRM_MODE_TUNNEL)
 		x->props.header_len += sizeof(struct iphdr);
+	else if (x->props.mode == XFRM_MODE_BEET)
+		x->props.header_len += IPV4_BEET_PHMAXLEN;
 	if (x->encap) {
 		struct xfrm_encap_tmpl *encap = x->encap;
 

net/ipv4/xfrm4_mode_beet.c

@@ -40,7 +40,7 @@ static int xfrm4_beet_output(struct xfrm_state *x, struct sk_buff *skb)
 	if (unlikely(optlen))
 		hdrlen += IPV4_BEET_PHMAXLEN - (optlen & 4);
 
-	skb_push(skb, x->props.header_len + hdrlen);
+	skb_push(skb, x->props.header_len - IPV4_BEET_PHMAXLEN + hdrlen);
 	skb_reset_network_header(skb);
 	top_iph = ip_hdr(skb);
 	skb->transport_header += sizeof(*iph) - hdrlen;