On Fri, 2016-10-21 at 14:57 +0000, Jon Maloy wrote: > > -----Original Message----- > > > > From: Ben Hutchings [mailto:b...@decadent.org.uk] > > Sent: Thursday, 20 October, 2016 12:40 > > > > To: Jon Maloy <jon.ma...@ericsson.com>; Ying Xue <ying.x...@gmail.com> > > > > > > Cc: netdev@vger.kernel.org; Qian Zhang <zhangqia...@360.cn>; Eric > > > > > > Dumazet > > > > <eduma...@google.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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