From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: [PATCH] Don't allow sharing of tx skbs on xen-netfront Date: Thu, 17 Nov 2011 20:17:01 +0000 Message-ID: <1321561021.8866.12.camel@dagon.hellion.org.uk> References: <1321298544-16434-1-git-send-email-nhorman@tuxdriver.com> <1321543238.3664.278.camel@zakaz.uk.xensource.com> <20111117192535.GB26847@hmsreliant.think-freely.org> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: "netdev@vger.kernel.org" , "David S. Miller" , Jeremy Fitzhardinge , Konrad Rzeszutek Wilk , "xen-devel@lists.xensource.com" To: Neil Horman Return-path: Received: from smtp.ctxuk.citrix.com ([62.200.22.115]:47396 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755266Ab1KQURE (ORCPT ); Thu, 17 Nov 2011 15:17:04 -0500 In-Reply-To: <20111117192535.GB26847@hmsreliant.think-freely.org> Sender: netdev-owner@vger.kernel.org List-ID: 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? Ian. > > Neil > > > Ian. > > > > > > > > Signed-off-by: Neil Horman > > > CC: "David S. Miller" > > > CC: Jeremy Fitzhardinge > > > CC: Konrad Rzeszutek Wilk > > > CC: xen-devel@lists.xensource.com > > > --- > > > drivers/net/xen-netfront.c | 6 ++++++ > > > 1 files changed, 6 insertions(+), 0 deletions(-) > > > > > > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c > > > index 226faab..fb1077b 100644 > > > --- a/drivers/net/xen-netfront.c > > > +++ b/drivers/net/xen-netfront.c > > > @@ -1252,6 +1252,12 @@ static struct net_device * __devinit xennet_create_dev(struct xenbus_device *dev > > > return ERR_PTR(-ENOMEM); > > > } > > > > > > + /* > > > + * Since frames remain on a queue after a return from xennet_start_xmit, > > > + * we can't support tx shared skbs > > > + */ > > > + netdev->priv_flags &= ~IFF_TX_SKB_SHARING; > > > + > > > np = netdev_priv(netdev); > > > np->xbdev = dev; > > > > > > > > > -- > > To unsubscribe from this list: send the line "unsubscribe netdev" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > >