Re: [Xen-devel] [PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Apr 30, 2013 at 07:06:01PM +0100, annie li wrote:
> 
> On 2013-4-15 13:06, Wei Liu wrote:
> > This patch tries to coalesce tx requests when constructing grant copy
> > structures. It enables netback to deal with situation when frontend's
> > MAX_SKB_FRAGS is larger than backend's MAX_SKB_FRAGS.
> >
> > With the help of coalescing, this patch tries to address two regressions and
> > avoid reopening the security hole in XSA-39.
> >
> > Regression 1. The reduction of the number of supported ring entries (slots)
> > per packet (from 18 to 17). This regression has been around for some time 
> > but
> > remains unnoticed until XSA-39 security fix. This is fixed by coalescing
> > slots.
> >
> > Regression 2. The XSA-39 security fix turning "too many frags" errors from
> > just dropping the packet to a fatal error and disabling the VIF. This is 
> > fixed
> > by coalescing slots (handling 18 slots when backend's MAX_SKB_FRAGS is 17)
> > which rules out false positive (using 18 slots is legit) and dropping 
> > packets
> > using 19 to `max_skb_slots` slots.
> >
> > To avoid reopening security hole in XSA-39, frontend sending packet using 
> > more
> > than max_skb_slots is considered malicious.
> >
> > The behavior of netback for packet is thus:
> >
> >      1-18            slots: valid
> >     19-max_skb_slots slots: drop and respond with an error
> >     max_skb_slots+   slots: fatal error
> >
> > max_skb_slots is configurable by admin, default value is 20.
> >
> > Also change variable name from "frags" to "slots" in netbk_count_requests.
> >
> > Please note that RX path still has dependency on MAX_SKB_FRAGS. This will be
> > fixed with separate patch.
> 
> Wei,
> 
> The RX path dependency on MAX_SKB_FRAGS in netback should not cause 
> packet dropping issue like TX path, RX path only calls netif_stop_queue 
> if RX ring is full.
> What is your plan on RX path? Is it in your schedule or you started it 
> already?
> 

Not yet planned or started. My general idea is that we need to remove
all dependency on MAX_SKB_FRAGS.

I'm working on bug fix for libxl now, so I might get to that later.


Wei.

> Thanks
> Annie

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