netdev - Re: Fwd: Simple kernel attack using socketpair. easy, 100% reproductiblle, works under guest. no way to protect :(

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Date:	Fri, 26 Nov 2010 09:59:11 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shan Wei <shanwei@...fujitsu.com>
Cc:	Марк Коренберг 
	<socketpair@...il.com>, David Miller <davem@...emloft.net>,
	netdev@...r.kernel.org
Subject: Re: Fwd: Simple kernel attack using socketpair. easy, 100%
 reproductiblle, works under guest. no way to protect :(

Le vendredi 26 novembre 2010 à 09:22 +0100, Eric Dumazet a écrit :
> Le vendredi 26 novembre 2010 à 15:41 +0800, Shan Wei a écrit :
> > Eric Dumazet wrote, at 11/25/2010 10:11 PM:
> > > @@ -1845,6 +1871,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
> > >  		unix_state_lock(sk);
> > >  		skb = skb_dequeue(&sk->sk_receive_queue);
> > >  		if (skb == NULL) {
> > > +			unix_sk(sk)->recursion_level = 0;
> > 
> > For SOCK_SEQPACKET type, no need to clear recursion_level counter?
> >  
> > 
> 
> There is no need actually to clear it at all.
> 
> If an application has a complex setup with a dependence tree of unix
> sockets, it will break if messages are not read fast enough.
> 
> So, maybe I should remove this line so that underlying problem comes
> into surface immediately, rather than while in stress load.
> 
> 

The whole sendfd feature is fundamentally flawed, since its not a "give
this file to another user", but "give a pointer to file structure"

As soon as you can pass af_unix sockets, you cannot know if the
intransit "refs to file structure" are going to be consumed by one or
other user. So a per user limit is not possible.

I am not sure it is fixable at all, unless adding a complete graph
structure between af_unix sockets that used the sendfd() mechanism.
(Its a NxN relationship... pretty hard to track)

Yes, we can add limits (global wide), but they could break legacy apps,
and a single user could lock in one fd all the tokens.



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