netdev - Re: Netlink mmap tx security?

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Message-ID: <20141217162646.GB28766@casper.infradead.org>
Date:	Wed, 17 Dec 2014 16:26:46 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, dborkman@...hat.com,
	luto@...capital.net, torvalds@...ux-foundation.org,
	kaber@...sh.net, netdev@...r.kernel.org
Subject: Re: Netlink mmap tx security?

On 12/16/14 at 04:02pm, Eric Dumazet wrote:
> On Tue, 2014-12-16 at 17:58 -0500, David Miller wrote:
> 
> > +		__skb_put(skb, nm_len);
> > +		memcpy(skb->data, (void *)hdr + NL_MMAP_HDRLEN, nm_len);
> > +		netlink_set_status(hdr, NL_MMAP_STATUS_UNUSED);
> >  
> 
> Not related to this patch, but it looks like netlink_set_status()
> barrier is wrong ?
> 
> It seems we need smp_wmb() after the memcpy() and before the
> hdr->nm_status = status; 

Yes, definitely wrong as-is. I'll send a patch. For the particular
case we'd need a smp_rmb() after the memcpy() to complete the loads.
The skb destructor needs a smp_wmb() after setting nm_len. We could
get away with a smp_wmb() first thing in netlink_set_status() with
the code as-is but smp_mb() might be the less fragile thing to do.
Objections?
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