netdev - Re: [PATCH] tcp: assign the sock correctly to an outgoing SYNACK packet

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Date:	Mon, 08 Apr 2013 10:36:26 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Paul Moore <pmoore@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	mvadkert@...hat.com
Subject: Re: [PATCH] tcp: assign the sock correctly to an outgoing SYNACK
 packet

On Mon, 2013-04-08 at 13:22 -0400, Paul Moore wrote:
> On Monday, April 08, 2013 12:14:34 PM David Miller wrote:
> > From: Paul Moore <pmoore@...hat.com>
> > Date: Mon, 08 Apr 2013 11:45:19 -0400
> > 
> > > Commit 90ba9b1986b5ac4b2d184575847147ea7c4280a2 converted
> > > tcp_make_synack() to use alloc_skb() directly instead of calling
> > > sock_wmalloc(), the goal being the elimination of two atomic
> > > operations.  Unfortunately, in doing so the change broke certain
> > > SELinux/NetLabel configurations by no longer correctly assigning
> > > the sock to the outgoing packet.
> > > 
> > > This patch fixes this regression by doing the skb->sk assignment
> > > directly inside tcp_make_synack().
> > > 
> > > Reported-by: Miroslav Vadkerti <mvadkert@...hat.com>
> > > Signed-off-by: Paul Moore <pmoore@...hat.com>
> > 
> > Setting skb->sk without the destructor results in an SKB that can live
> > potentially forever with a stale reference to a destroyed socket.
> > 
> > You cannot fix the problem in this way.
> 
> Okay, no worries, I'll work on v2.  For some reason I missed the destructor 
> assignment in skb_set_owner_w(); I guess I was spending so much time hunting 
> around looking for the missing skb->sk assignment that once I found it I 
> declared victory ... a bit too soon.
> 
> Looking at the code again, I think the right solution is to call 
> skb_set_owner_w() instead of doing the assignment directly but that is 
> starting to bring us back to sock_wmalloc(force == 1) which gets back to 
> Eric's comments ... (below) ...
> 
> On Monday, April 08, 2013 09:19:23 AM Eric Dumazet wrote:
> > Keeping a pointer on a socket without taking a refcount is not going to
> > work.
> > 
> > We are trying to make the stack scale, so you need to add a selinux call
> > to take a ref count only if needed.
> > 
> > That is : If selinux is not used, we don't need to slow down the stack.
> 
> Contrary to popular belief, my goal is to not destroy the scalability and/or 
> performance of our network stack, I just want to make sure we have a quality 
> network stack that is not only fast and scalable, but also preserves the 
> security functionality that makes Linux attractive to a number of users.  To 
> that end, we could put a #ifdef in the middle of tcp_make_synack(), but that 
> seems very ugly to me and I think sets a bad precedence for the network stack 
> and kernel as a whole.
> 
> So a question for Dave, et al. - would you prefer that I fix this by:
> 
> 1. Restore the original sock_wmalloc() call?
> 2. Keep things as-is with skb_alloc() but add skb_set_owner_w()?
> 3. Add a #ifdef depending on SELinux (probably the LSM in general to be safe) 
> and use sock_wmalloc() if enabled, skb_alloc() if not?

Didnt we had the same issue with RST packets ?

Please take a look at commit 3a7c384ffd57ef5fbd95f48edaa2ca4eb3d9f2ee



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