[Xen-devel] Re: [PATCH] Don't allow sharing of tx skbs on xen-netfront

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2011-11-17 at 20:45 +0000, Neil Horman wrote:
> On Thu, Nov 17, 2011 at 08:17:01PM +0000, Ian Campbell wrote:
> > On Thu, 2011-11-17 at 19:25 +0000, Neil Horman wrote:
> > > On Thu, Nov 17, 2011 at 03:20:38PM +0000, Ian Campbell wrote:
> > > > On Mon, 2011-11-14 at 14:22 -0500, Neil Horman wrote:
> > > > > It was pointed out to me recently that the xen-netfront driver can't 
> > > > > safely
> > > > > support shared skbs on transmit, since, while it doesn't maintain skb 
> > > > > state
> > > > > directly, it does pass a pointer to the skb to the hypervisor via a 
> > > > > list, and
> > > > > the hypervisor may expect the contents of the skb to remain stable.  
> > > > > Clearing
> > > > > the IFF_TX_SKB_SHARING flag after the call to alloc_etherdev to make 
> > > > > it safe.
> > > > 
> > > > What are the actual constraints here? The skb is used as a handle to the
> > > > skb->data and shinfo (frags) and to complete at the end. It's actually
> > > > those which are passed to the hypervisor (effectively the same as
> > > > passing those addresses to the h/w for DMA).
> > > > 
> > > > Which parts of the skb are expected/allowed to not remain stable?
> > > > 
> > > > (Appologies if the above seems naive, I seem to have missed the
> > > > introduction of shared tx skbs and IFF_TX_SKB_SHARING)
> > > > 
> > > Its ok, this is the most accurate description from the previous threads 
> > > on the
> > > subject:
> > > 2
> > > 
> > > The basic problem boils down the notion that some drivers, when they 
> > > receive an
> > > skb in their xmit paths, presume to have sole ownership of the skb, and 
> > > as a
> > > result may do things like add the skb to a list, or otherwise store 
> > > stateful
> > > data in the skb.  If the skb is shared, thats unsafe to do, as the stack 
> > > still
> > > holds a reference to the skb, and make make changes without serializing 
> > > them
> > > against the driver.  So we have to flag those drivers which preform these 
> > > kinds
> > > of actions.  xen-netfront doesn't strictly speaking modify any state 
> > > directly ni
> > > an skb, but it does place a pointer to the skb in a data structure here:
> > > 
> > > np->tx_skbs[id].skb = skb;
> > > 
> > > Which then gets handed off to the hypervisior.  Since the hypervisor now 
> > > has
> > > access to that skb pointer, and we can't be sure (from the guest 
> > > perspective),
> > > what it does with that information, it would be better to be safe by 
> > > disallowing
> > > shared skbs in this path.
> > 
> > The skb pointer itself doesn't get given to the backend/hypervisor. The
> > page which skb->data refers to is granted to the backend domain, as are
> > the pages in the frags.
> > 
> > I think we only need to be sure that the frontend doesn't rely on
> > anything in the skb itself, right? Does skb->data or shinfo count from
> > that perspective?
> shinfo is definately a problem, as other devices may make modifications to it.
> skb->data is probably safer, but is also potentially suspect (for instance if
> another device appends an additional header to the data for instance)

A device is allowed to rely on these things being stable while in its
start_xmit, right? (otherwise I don't see how any device can ever
cope...).

netfront only uses shinfo and ->data during start_xmit in order to
create the necessary grant reference (which can be thought of as a DMA
address passed to the virtual hardware). The only use of the stashed skb
pointer outside of this are to dev_kfree_skb on tx completion (from
either tx_buf_gc (normal completion) or release_tx_buf ("hardware"
reset).

Ian.


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