netdev - Re: [PATCH net-next ] mpls: In mpls_egress verify the packet length.

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Message-Id: <20150312.182558.447886504045770209.davem@davemloft.net>
Date:	Thu, 12 Mar 2015 18:25:58 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ebiederm@...ssion.com
Cc:	rshearma@...cade.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next ] mpls: In mpls_egress verify the packet
 length.

From: ebiederm@...ssion.com (Eric W. Biederman)
Date: Thu, 12 Mar 2015 15:54:52 -0500

> Robert Shearman noticed that mpls_egress is failing to verify that
> the bytes to be examined are in fact present in the packet before
> mpls_egress reads those bytes.
> 
> Reported-by: Robert Shearman <rshearma@...cade.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Let's see if we can consolidate these pskb_may_pull() calls into one,
in the fast path they are relatively cheap but not zero cost.

The caller (mpls_forward()) is doing one as well.

All you care about is the version field and the hop_limit/ttl.

For ipv4, version is in byte 0, and ttl is in byte 9.

For ipv6, version is also in byte zero and hop_limit is in byte 7.

Therefore if you do pskb_may_pull(skb, 10) it will cover all cases
whilst not exceeding the size of either protocol's total header.

Above the pskb_may_pull() call can add a big comment explaining what
you are doing and why this is sufficient.
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