Patch "ipv6: avoid write to a possibly cloned skb" has been added to the 4.9-stable tree

Patch "ipv6: avoid write to a possibly cloned skb" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    ipv6: avoid write to a possibly cloned skb

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ipv6-avoid-write-to-a-possibly-cloned-skb.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From foo@baz Sat Mar 18 22:03:25 CST 2017
From: Florian Westphal <fw@strlen.de>
Date: Mon, 13 Mar 2017 16:24:28 +0100
Subject: ipv6: avoid write to a possibly cloned skb

From: Florian Westphal <fw@strlen.de>


[ Upstream commit 79e49503efe53a8c51d8b695bedc8a346c5e4a87 ]

ip6_fragment, in case skb has a fraglist, checks if the
skb is cloned.  If it is, it will move to the 'slow path' and allocates
new skbs for each fragment.

However, right before entering the slowpath loop, it updates the
nexthdr value of the last ipv6 extension header to NEXTHDR_FRAGMENT,
to account for the fragment header that will be inserted in the new
ipv6-fragment skbs.

In case original skb is cloned this munges nexthdr value of another
skb.  Avoid this by doing the nexthdr update for each of the new fragment
skbs separately.

This was observed with tcpdump on a bridge device where netfilter ipv6
reassembly is active:  tcpdump shows malformed fragment headers as
the l4 header (icmpv6, tcp, etc). is decoded as a fragment header.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Reported-by: Andreas Karis <akaris@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv6/ip6_output.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -757,13 +757,14 @@ slow_path:
 	 *	Fragment the datagram.
 	 */
 
-	*prevhdr = NEXTHDR_FRAGMENT;
 	troom = rt->dst.dev->needed_tailroom;
 
 	/*
 	 *	Keep copying data until we run out.
 	 */
 	while (left > 0)	{
+		u8 *fragnexthdr_offset;
+
 		len = left;
 		/* IF: it doesn't fit, use 'mtu' - the data space left */
 		if (len > mtu)
@@ -808,6 +809,10 @@ slow_path:
 		 */
 		skb_copy_from_linear_data(skb, skb_network_header(frag), hlen);
 
+		fragnexthdr_offset = skb_network_header(frag);
+		fragnexthdr_offset += prevhdr - skb_network_header(skb);
+		*fragnexthdr_offset = NEXTHDR_FRAGMENT;
+
 		/*
 		 *	Build fragment header.
 		 */


Patches currently in stable-queue which might be from fw@strlen.de are

queue-4.9/ipv6-avoid-write-to-a-possibly-cloned-skb.patch
queue-4.9/bridge-drop-netfilter-fake-rtable-unconditionally.patch