From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH 1/2] virtio-net: Verify page list size before fitting
 into skb
Date: Tue, 27 Sep 2011 15:37:20 +0300
Message-ID: <20110927123719.GB11276@redhat.com>
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Cc: linux-kernel@vger.kernel.org,
	Rusty Russell <rusty@rustcorp.com.au>,
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To: Sasha Levin <levinsasha928@gmail.com>
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On Tue, Sep 27, 2011 at 02:20:29PM +0300, Sasha Levin wrote:
> On Tue, 2011-09-27 at 10:00 +0300, Michael S. Tsirkin wrote:
> > >               skb = page_to_skb(vi, page, len);
> > >               ...
> > 
> > Sorry I don't get it yet. Where is mergeable ignored here?
> 
> The NULL deref happens in page_to_skb(), before merge buffers are
> handled.

The len here is a single buffer length, so for mergeable
buffers it is <= the size of the buffer we gave to hardware,
which is PAGE_SIZE.

        err = virtqueue_add_buf_single(vi->rvq, page_address(page),
                                       PAGE_SIZE, false, page, gfp);
 

Unless of course we are trying to work around broken hardware again,
which I don't have a problem with, but should probably
get appropriate comments in code and trigger a warning.

> I'll test it and see if it's really the case.
> 
> -- 
> 
> Sasha.