netdev - Re: [PATCH 1/2] virtio-net: Verify page list size before fitting into skb

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Date:	Mon, 26 Sep 2011 22:55:41 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] virtio-net: Verify page list size before fitting
 into skb

On Mon, Sep 26, 2011 at 10:37:22PM +0300, Sasha Levin wrote:
> On Mon, 2011-09-26 at 21:44 +0300, Michael S. Tsirkin wrote:
> > On Mon, Sep 26, 2011 at 08:41:08PM +0300, Sasha Levin wrote:
> > > This patch verifies that the length of a buffer stored in a linked list
> > > of pages is small enough to fit into a skb.
> > > 
> > > If the size is larger than a max size of a skb, it means that we shouldn't
> > > go ahead building skbs anyway since we won't be able to send the buffer as
> > > the user requested.
> > > 
> > > Cc: Rusty Russell <rusty@...tcorp.com.au>
> > > Cc: "Michael S. Tsirkin" <mst@...hat.com>
> > > Cc: virtualization@...ts.linux-foundation.org
> > > Cc: netdev@...r.kernel.org
> > > Cc: kvm@...r.kernel.org
> > > Signed-off-by: Sasha Levin <levinsasha928@...il.com>
> > 
> > Interesting.  This is a theoretical issue, correct?
> > Not a crash you actually see.
> 
> Actually it was an actual crash caused when our virtio-net driver in kvm
> tools did funny things and passed '(u32)-1' length as a buffer length to
> the guest kernel.
> 
> > This crash would mean device is giving us packets
> > that are way too large. Avoiding crashes even in the face of
> > a misbehaved device is a good idea, but should
> > we print a diagnostic to a system log?
> > Maybe rate-limited or print once to avoid filling
> > up the disk. Other places in driver print with pr_debug
> > I'm not sure that's right but better than nothing.
> 
> Yup, I'll add some debug info.
> 
> > > ---
> > >  drivers/net/virtio_net.c |    3 +++
> > >  1 files changed, 3 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 0c7321c..64e0717 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -165,6 +165,9 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> > >  	unsigned int copy, hdr_len, offset;
> > >  	char *p;
> > >  
> > > +	if (len > MAX_SKB_FRAGS * PAGE_SIZE)
> > 
> > unlikely()?
> > 
> > Also, this seems too aggressive: at this point len includes the header
> > and the linear part. The right place for this
> > test is probably where we fill in the frags, just before
> > while (len)
> > 
> > The whole can only happen when mergeable buffers
> > are disabled, right?
> 
> >From what I understand it can happen whenever you're going to build a
> skb longer than PAGE_SIZE.

Hmm how exactly?  With mergeable buffers this only gets
the length of the 1st chunk which is up to 4K unless the driver
is buggy ...

> -- 
> 
> Sasha.
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