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Re: [Xen-devel] Moving frags and SKBTX_DEV_ZEROCOPY skbs

On 14/05/14 15:23, Eric Dumazet wrote:
On Wed, 2014-05-14 at 14:40 +0100, Zoltan Kiss wrote:

Recently I've investigated issues around SKBTX_DEV_ZEROCOPY skbs where
the frags list were modified. I came across this function skb_shift(),
which moves frags between skbs. And there are a lot more of such kind,
skb_split or skb_try_coalesce, for example.
It could be a dangerous thing if a frag is referenced from an skb which
doesn't have the original destructor_arg, and to avoid that
skb_orphan_frags should be called. Although probably these functions are
not normally touched in usual usecases, I think it would be useful to
review core skb functions proactively and add an skb_orphan_frags
everywhere where the frags could be referenced from other places.
Any opinion about this?

For skb_shift(), it is currently used from tcp stack only, where
this SKBTX_DEV_ZEROCOPY thing is not used, so I do not think there is a
bug for the moment.
It is called from tcp_input.c, which suggests it can be called on incoming TCP packets. If the backend domain communicates with the frontend through sockets, zerocopy packets can turn up here. But here is the thing: deliver_skb calls orphan_frags for every packet delivered to the local stack, so we are safe IF these functions are called before the IP stack. So we are safe now, but things can go wrong, if: - such a frag-mangling function is called before deliver_skb, now or in the future
- if someone wants to take advantage of zerocopy in the guest<->backend path

I already gave a patch for skb_try_coalesce() : For this one we do not
wan skb_orphan_frags() overhead. Its simply better in this case to

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 1b62343f5837..85995a14aafc 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3838,7 +3839,10 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff 
                return true;

-       if (skb_has_frag_list(to) || skb_has_frag_list(from))
+       if (skb_has_frag_list(to) ||
+           skb_has_frag_list(from) ||
+           (skb_shinfo(to)->tx_flags & SKBTX_DEV_ZEROCOPY) ||
+           (skb_shinfo(from)->tx_flags & SKBTX_DEV_ZEROCOPY))
                return false;

        if (skb_headlen(from) != 0) {

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