[PATCH] ipv6: sr: fix get_srh() to comply with IPv6 standard "RFC 8200"

IPv6 packet may carry more than one extension header, and IPv6 nodes must
acceptand attempt to process extension headers in any order and occurring
any number of times in the same packet. Hence, there should be no
assumption that Segment Routing extension header is to appear immediately
after the IPv6 header.

Moreover, section 4.1 of RFC 8200 gives a recommendation on the order of
appearance of those extension headers within an IPv6 packet. According to
this recommendation, Segment Routing extension header should appear after
Hop-by-Hop and Destination Options headers (if they present).

This patch fixes the get_srh(), so it gets the segment routing header
regardless of its position in the chain of the extension headers in IPv6
packet, and makes sure that the IPv6 routing extension header is of
Type 4.

Signed-off-by: Ahmed Abdelsalam <redacted>
---
 net/ipv6/seg6_local.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 9c1a885..ae97fd9 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c

@@ -62,17 +62,19 @@ static struct seg6_local_lwt *seg6_local_lwtunnel(struct lwtunnel_state *lwt)
 static struct ipv6_sr_hdr *get_srh(struct sk_buff *skb)
 {
 	struct ipv6_sr_hdr *srh;
-	struct ipv6hdr *hdr;
-	int len;
+	int len, srhoff;
 
-	hdr = ipv6_hdr(skb);
-	if (hdr->nexthdr != IPPROTO_ROUTING)
+	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
 		return NULL;
 
-	srh = (struct ipv6_sr_hdr *)(hdr + 1);
-	len = (srh->hdrlen + 1) << 3;
+	srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
 
-	if (!pskb_may_pull(skb, sizeof(*hdr) + len))
+	/* make sure it's a Segment Routing header (Routing Type 4) */
+	if (srh->type != IPV6_SRCRT_TYPE_4)
+		return NULL;
+
+	len = (srh->hdrlen + 1) << 3;
+	if (!pskb_may_pull(skb, srhoff + len))
 		return NULL;
 
 	if (!seg6_validate_srh(srh, len))

-- 
2.7.4

On 08/28/2017 07:20 PM, Ahmed Abdelsalam wrote:

This patch fixes the get_srh(), so it gets the segment routing header
regardless of its position in the chain of the extension headers in IPv6
packet, and makes sure that the IPv6 routing extension header is of
Type 4.

Ahmed,

You need to initialize srhoff to 0, otherwise ipv6_find_hdr() will crash
the kernel by dereferencing an uninitialized pointer.

Please test your patches before submitting them.

Furthermore, your pskb_may_pull() check should happen right after the
call to ipv6_find_hdr, with srhoff + sizeof(*srh) as argument. Once you
have checked the SRH type, you can then do another pskb_may_pull with
srhoff + len.

David

On Mon, 28 Aug 2017 19:48:15 +0100
David Lebrun [off-list ref] wrote:

On 08/28/2017 07:20 PM, Ahmed Abdelsalam wrote:

quoted

This patch fixes the get_srh(), so it gets the segment routing header
regardless of its position in the chain of the extension headers in IPv6
packet, and makes sure that the IPv6 routing extension header is of
Type 4.

Ahmed,

You need to initialize srhoff to 0, otherwise ipv6_find_hdr() will crash
the kernel by dereferencing an uninitialized pointer.

Please test your patches before submitting them.

Furthermore, your pskb_may_pull() check should happen right after the
call to ipv6_find_hdr, with srhoff + sizeof(*srh) as argument. Once you
have checked the SRH type, you can then do another pskb_may_pull with
srhoff + len.

David

Thanks David, 
 
I will address the comments and re-submit the patch after testing.

-- 
Ahmed