Re: FreeBSD 5 ip_gre and netisr_enable=1
From: Max Laier (max_at_love2party.net)
Date: 08/25/05
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To: freebsd-net@freebsd.org Date: Thu, 25 Aug 2005 23:00:58 +0200
On Thursday 25 August 2005 22:10, ming fu wrote:
> Hi,
>
> This problem exit in some old gre.c (not a part of official freebsd) to
> handle wccp packets. A carefully crafted packet can cause it to deplete
> kernel stack and casuing a panic. It can crash a 4.2 kernel with about
> 200-300 repeated ip+gre header.
>
> I believe the problem appears on FreeBSD 5 with ip_gre() and
> net.isr.enable = 1. It probably easier to crash a 5.x because more calls
> are involved in FreeBSD 5 than 4.x, thus more stack can be consumed with
> the same repetition of headers.
>
> when a GRE packet gets into the ip_gre2(), its gre header is stripped
> and sent to netisr_dispatch() for ip_input() processing again. In case,
> the net.isr.enable is 1, the packet will be delivered to ip_input
> directly instead of put in the queue.
>
> If someone create a packet consists of repeated ip and gre header,
>
> ip hdr : gre hdr : ip hdr : gre hdr : ...... repeat a few
> hundred times.
>
> it can cause a loop around
> ip_gre->ip_gre2->netisr_dispatch->ip_input->ip_gre ..., not too
> difficult to deplete the kernel stack.
>
> It only takes 24 bytes to force the kernel to go one round through these
> calls.
>
> Any suggestion of how to fix this?
>
> send the gre stripped packet to netisr_queue() is an easy, albeit slow
> solution.
>
> I fix the older gre.c file by making sure the inner packet is not a GRE
> before deliver to ip_input. However, it was ugly to parse the inner
> header of in ip_gre2().
You could use an mbuf_tag to keep track of recursion in the same way it is
done in gif. There is certainly some overhead involved as well, however.
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