xfrm: BEET mode doesn't support fragments for inner packets

BEET mode replaces the IP(6) Headers with new IP(6) Headers when sending
packets. However, when it's a fragment before the replacement, currently
kernel keeps the fragment flag and replace the address field then encaps
it with ESP. It would cause in RX side the fragments to get reassembled
before decapping with ESP, which is incorrect.

In Xiumei's testing, these fragments went over an xfrm interface and got
encapped with ESP in the device driver, and the traffic was broken.

I don't have a good way to fix it, but only to warn this out in dmesg.
Reported-by: Xiumei Mu <xmu@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_output.c

@@ -660,6 +660,12 @@ static int xfrm4_extract_output(struct xfrm_state *x, struct sk_buff *skb)
 {
 	int err;
 
+	if (x->outer_mode.encap == XFRM_MODE_BEET &&
+	    ip_is_fragment(ip_hdr(skb))) {
+		net_warn_ratelimited("BEET mode doesn't support inner IPv4 fragments\n");
+		return -EAFNOSUPPORT;
+	}
+
 	err = xfrm4_tunnel_check_size(skb);
 	if (err)
 		return err;
@@ -705,8 +711,15 @@ static int xfrm6_tunnel_check_size(struct sk_buff *skb)
 static int xfrm6_extract_output(struct xfrm_state *x, struct sk_buff *skb)
 {
 #if IS_ENABLED(CONFIG_IPV6)
+	unsigned int ptr = 0;
 	int err;
 
+	if (x->outer_mode.encap == XFRM_MODE_BEET &&
+	    ipv6_find_hdr(skb, &ptr, NEXTHDR_FRAGMENT, NULL, NULL) >= 0) {
+		net_warn_ratelimited("BEET mode doesn't support inner IPv6 fragments\n");
+		return -EAFNOSUPPORT;
+	}
+
 	err = xfrm6_tunnel_check_size(skb);
 	if (err)
 		return err;