esp4: Fix udpencap for local TCP packets.

[ Upstream commit 0e78a873 ]

Locally generated TCP packets are usually cloned, so we
do skb_cow_data() on this packets. After that we need to
reload the pointer to the esp header. On udpencap this
header has an offset to skb_transport_header, so take this
offset into account.

This is a backport of:
commit 0e78a873 ("esp4: Fix udpencap for local TCP packets.")

Fixes: 67d349ed ("net/esp4: Fix invalid esph pointer crash")
Fixes: fca11ebd ("esp4: Reorganize esp_output")
Reported-by: Don Bowman <db@donbowman.ca>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv4/esp4.c

@@ -212,6 +212,7 @@ static int esp_output(struct xfrm_state *x, struct sk_buff *skb)
 	u8 *iv;
 	u8 *tail;
 	u8 *vaddr;
+	int esph_offset;
 	int blksize;
 	int clen;
 	int alen;
@@ -392,12 +393,14 @@ static int esp_output(struct xfrm_state *x, struct sk_buff *skb)
 	}
 
 cow:
+	esph_offset = (unsigned char *)esph - skb_transport_header(skb);
+
 	err = skb_cow_data(skb, tailen, &trailer);
 	if (err < 0)
 		goto error;
 	nfrags = err;
 	tail = skb_tail_pointer(trailer);
-	esph = ip_esp_hdr(skb);
+	esph = (struct ip_esp_hdr *)(skb_transport_header(skb) + esph_offset);
 
 skip_cow:
 	esp_output_fill_trailer(tail, tfclen, plen, proto);