Message-ID: <A2BAEFC30C8FD34388F02C9B3121859D2259D55C@eusaamb103.ericsson.se>
Date:   Tue, 1 Nov 2016 11:36:47 +0000
From:   Jon Maloy <jon.maloy@...csson.com>
To:     张谦 <zhangqian-c@....cn>,
        Ben Hutchings <ben@...adent.org.uk>,
        Ying Xue <ying.xue0@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH net] tipc: Guard against tiny MTU in tipc_msg_build()

Hi,
I think we all agreed in the end that this is a possible, but highly implausible, scenario, and rather as a point of exploit than a functional bug.
The solution is very simple, and described further down in this mail thread. I have not done anything to it yet, but you are welcome to contribute.

BR
///jon


> -----Original Message-----
> From: 张谦 [mailto:zhangqian-c@....cn]
> Sent: Tuesday, 01 November, 2016 02:35
> To: Ben Hutchings <ben@...adent.org.uk>; Jon Maloy
> <jon.maloy@...csson.com>; Ying Xue <ying.xue0@...il.com>
> Cc: netdev@...r.kernel.org; Eric Dumazet <edumazet@...gle.com>
> Subject: Re: [PATCH net] tipc: Guard against tiny MTU in tipc_msg_build()
> 
> Hi all,
> I have accomplished a PoC can help you to confirm this issue.
> 
> And two weeks passed from the last mail, can you tell me the progress of the
> patch to this flaw?
> 
> Thanks.
> 
> Qian Zhang
> Marvel Team Qihoo 360
> 
> 
> -----邮件原件-----
> 发件人: Ben Hutchings [mailto:ben@...adent.org.uk]
> 发送时间: 2016年10月21日 23:00
> 收件人: Jon Maloy; Ying Xue
> 抄送: netdev@...r.kernel.org; 张谦; Eric Dumazet
> 主题: 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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