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

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Message-ID: <A2BAEFC30C8FD34388F02C9B3121859D22587ADA@eusaamb103.ericsson.se>
Date:   Thu, 20 Oct 2016 14:51:05 +0000
From:   Jon Maloy <jon.maloy@...csson.com>
To:     Ben Hutchings <ben@...adent.org.uk>, 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()



> -----Original Message-----
> From: Ben Hutchings [mailto:ben@...adent.org.uk]
> Sent: Thursday, 20 October, 2016 08:46
> 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.

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.

Regards
///jon

> 
> 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.