Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice

[Shirley Ma <mashirle@us.ibm.com>]

On Wed, 2011-05-18 at 18:47 +0300, Michael S. Tsirkin wrote:
> On Wed, May 18, 2011 at 07:38:27AM -0700, Shirley Ma wrote:
> > On Wed, 2011-05-18 at 13:40 +0200, Michał Mirosław wrote:
> > > >> >> Not more other restrictions, skb clone is OK.
> pskb_expand_head()
> > > looks
> > > >> >> OK to me from code review.
> > > >> > Hmm. pskb_expand_head calls skb_release_data while keeping
> > > >> > references to pages. How is that ok? What do I miss?
> > > >> It's making copy of the skb_shinfo earlier, so the pages
> refcount
> > > >> stays the same.
> > > > Exactly. But the callback is invoked so the guest thinks it's ok
> to
> > > > change this memory. If it does a corrupted packet will be sent
> out.
> > > 
> > > Hmm. I tool a quick look at skb_clone(), and it looks like this
> > > sequence will break this scheme:
> > > 
> > > skb2 = skb_clone(skb...);
> > > kfree_skb(skb) or pskb_expand_head(skb);  /* callback called */
> > > [use skb2, pages still referenced]
> > > kfree_skb(skb); /* callback called again */
> > > 
> > > This sequence is common in bridge, might be in other places.
> > > 
> > > Maybe this ubuf thing should just track clones? This will make it
> work
> > > on all devices then.
> > 
> > The callback was only invoked when last reference of skb was gone.
> > skb_clone does increase skb refcnt. I tested tcpdump on lower
> device, it
> > worked.
> 
> Right, it will normally work, but two issues I think you miss:
> 1. malicious guest can change the memory between when it is sent out
> by
>    device and consumed by tcpdump, so you will see different things
>    (not sure how important this is).
> 2. if tcpdump stops consuming stuff from the packet socket (it's
>    userspace, can't be trusted) then we won't get a callback for
>    page potentially forever, guest networking will get blocked etc.
> > For the sequence of:
> > 
> > skb_clone  -> last refcnt + 1
> > kfree_skb() or pskb_expand_head -> callback not called
> > kfree_skb() -> callback called
> > 
> > I will check page refcount to see whether it's balanced.
> > 
> > Thanks
> > shirley
> 
> 
> pskb_expand_head is a problem anyway I think as it
> can hang on to pages after it calls release_data.
> Then guest will modify these pages and you get trash there. 

This can be avoid by allowing pskb_expand_head in fastpath only, I
think. But not sure whether tcpdump can still work with this.

Thanks
Shirley