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Re: [Xen-devel] [PATCH net-next] xen-netback: Stop using xenvif_tx_pending_slots_available

On 21/03/14 09:36, Ian Campbell wrote:
On Thu, 2014-03-20 at 19:32 +0000, Zoltan Kiss wrote:
Since the early days TX stops if there isn't enough free pending slots to
consume a maximum sized (slot-wise) packet. Probably the reason for that is to
avoid the case when we don't have enough free pending slot in the ring to finish
the packet. But if we make sure that the pending ring has the same size as the
shared ring, that shouldn't really happen. The frontend can only post packets
which fit the to the free space of the shared ring. If it doesn't, the frontend
has to stop, as it can only increase the req_prod when the whole packet fits
onto the ring.

My only real concern here is that by removing these checks we are
introducing a way for a malicious or buggy guest to trigger misbehaviour
in the backend, leading to e.g. a DoS.

Should we need to some sanity checks which shutdown the ring if
something like this occurs? i.e. if we come to consume a packet and
there is insufficient space on the pending ring we kill the vif.
The backend doesn't see what the guest does with the responses, and that's OK, it's the guest's problem, after netback increased rsp_prod_pvt it doesn't really care. But as soon as the guest start placing new requests after rsp_prod_pvt, or just increasing req_prod so req_prod-rsp_prod_pvt > XEN_NETIF_TX_RING_SIZE, it becomes an issue. So far this xenvif_tx_pending_slots_available indirectly saved us from consuming requests overwriting still pending requests, but the guest could still overwrote our responses. But again, that's still the guests problem, we had the original request saved in the pending ring data. If the guest went too far, build_gops killed the vif when req_prod-req_cons > XEN_NETIF_TX_RING_SIZE Indirect above means it only happened because the pending ring had the same size, and the used slots has a 1-to-1 mapping for slots between rsp_prod_pvt and req_cons. So xenvif_tx_pending_slots_available also means (req_cons - rsp_prod_pvt) + XEN_NETBK_LEGACY_SLOTS_MAX < XEN_NETIF_TX_RING_SIZE (does this look familiar? :)
But consuming overrunning requests after rsp_prod_pvt is a problem:
- NAPI instance races with dealloc thread over the slots. The first reads them as requests, the second writes them as responses - the NAPI instance overwrites used pending slots as well, so skb frag release go wrong etc. But the current RING_HAS_UNCONSUMED_REQUESTS fortunately saves us here, let me explain it through an example:
rsp_prod_pvt = 0
req_cons = 253
req_prod = 258

req = 5
rsp = 3

So in xenvif_tx_build_gops work_to_do will be 3, and in xenvif_count_requests we bail out when we see that the packet actually exceeds that. So in the end, we are safe here, but we shouldn't change that macro I suggested to refactor :)


What's the invariant we are relying on here, is it:
     req_prod >= req_cons >= pending_prod >= pending_cons >= rsp_prod >= 

This patch avoid using this checking, makes sure the 2 ring has the same size,
and remove a checking from the callback. As now we don't stop the NAPI instance
on this condition, we don't have to wake it up if we free pending slots up.

Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
index bef37be..a800a8e 100644
--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h
@@ -81,7 +81,7 @@ struct xenvif_rx_meta {


-#define MAX_PENDING_REQS 256

XEN_NETIF_TX_RING_SIZE is already == 256, right? (Just want to make sure
this is semantically no change).
Yes, it is __CONST_RING_SIZE(xen_netif_tx, PAGE_SIZE)

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