netdev - Re: [PATCH net] tipc: Guard against tiny MTU in tipc_msg

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Message-ID: <1477062005.2670.5.camel@decadent.org.uk>
Date:   Fri, 21 Oct 2016 16:00:05 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Jon Maloy <jon.maloy@...csson.com>, Ying Xue <ying.xue0@...il.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Qian Zhang <zhangqian-c@....cn>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] tipc: Guard against tiny MTU in tipc_msg_build()

On Fri, 2016-10-21 at 14:57 +0000, Jon Maloy wrote:
> > -----Original Message-----
> > > > From: Ben Hutchings [mailto:ben@...adent.org.uk]
> > Sent: Thursday, 20 October, 2016 12:40
> > > > To: Jon Maloy <jon.maloy@...csson.com>; Ying Xue <ying.xue0@...il.com>
> > > > > > Cc: netdev@...r.kernel.org; Qian Zhang <zhangqian-c@....cn>; Eric Dumazet
> > > > <edumazet@...gle.com>
> > Subject: Re: [PATCH net] tipc: Guard against tiny MTU in tipc_msg_build()
> > 
> > On Thu, 2016-10-20 at 14:51 +0000, Jon Maloy wrote:
> > [...]
> > > > At this point we're about to copy INT_H_SIZE + mhsz bytes into the
> > > > first fragment.  If that's already limited to be less than or equal to
> > > > MAX_H_SIZE, comparing with MAX_H_SIZE would be fine.  But if
> > 
> > MAX_H_SIZE
> > > > is the maximum value of mhsz, that won't be good enough.
> > > 
> > > 
> > > 
> > > MAX_H_SIZE is 60 bytes, but in practice you will never see an mhsz larger than
> > 
> > the biggest header we are actually using, which is MCAST_H_SIZE (==44 bytes).
> > > INT_H_SIZE is 40 bytes, so you are in reality testing for whether we have an mtu
> > 
> > < 84 bytes.
> > > You won't find any interfaces or protocols that come even close to this
> > 
> > limitation, so to me this test is redundant.
> > 
> > But I can easily create such an interface:
> > 
> > $ unshare -n -U -r
> > # ip l set lo mtu 1
> > 
> > Ben.
> 
> 
> It won't be very useful though. But I assume you mean it could be a
> possible exploit,

Exactly.

>  and I suspect a few other things would break both in TIPC and in
> other stacks if you do anything like that. I think the solution to
> this is not to fix all possible places in the code where this can go
> wrong, but rather to have a generic test where we refuse to attach
> bearers/interfaces offering an mtu < e.g. 1000 bytes. This can easily
> be done in tipc_enable_l2_media().

Yes.

Ben.

-- 
Ben Hutchings
One of the nice things about standards is that there are so many of
them.


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