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

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Date:   Thu, 20 Oct 2016 13:46:25 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Ying Xue <ying.xue0@...il.com>, Jon Maloy <jon.maloy@...csson.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 17:30 +0800, Ying Xue wrote:
> On 10/19/2016 10:16 AM, Ben Hutchings wrote:
> > Qian Zhang (张谦) reported a potential socket buffer overflow in
> > tipc_msg_build().  The minimum fragment length needs to be checked
> > against the maximum packet size, which is based on the link MTU.
[...]
> >  
> > --- a/net/tipc/msg.c
> > +++ b/net/tipc/msg.c
> > @@ -274,6 +274,10 @@ int tipc_msg_build(struct tipc_msg *mhdr, struct msghdr *m,
> > > >  		goto error;
> > > >  	}
> >  
> > > > +	/* Check that fragment and message header will fit */
> > > > +	if (INT_H_SIZE + mhsz > pktmax)
> > +		return -EMSGSIZE;
> 
> 
> The "mhsz" represents the size of tipc packet header for current socket,
> INT_H_SIZE indicates the size of tipc internal message header. So it
> seems unreasonable to identify whether the sum of both header sizes is
> bigger than MTU size. In my opinion, it's better to use MAX_H_SIZE to
> compare it with pktmax. If MAX_H_SIZE is bigger than pktmax, we should
> return EMSGSIZE error code.

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.

Ben.

> > +
> > > >  	/* Prepare reusable fragment header */
> > > >  	tipc_msg_init(msg_prevnode(mhdr), &pkthdr, MSG_FRAGMENTER,
> > > >  		      FIRST_FRAGMENT, INT_H_SIZE, msg_destnode(mhdr));
> > 
> 
> 
> 
-- 
Ben Hutchings
Never put off till tomorrow what you can avoid all together.


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