[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] Re: [PATCH] Don't allow sharing of tx skbs on xen-netfront
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: > > http://lists.openwall.net/netdev/2011/08/22/63 > > > > 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) Neil _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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