> -----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, 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(). ///jon > > -- > Ben Hutchings > Never put off till tomorrow what you can avoid all together.
