On Tue, Apr 30, 2013 at 07:06:01PM +0100, annie li wrote:

On 2013-4-15 13:06, Wei Liu wrote:

quoted

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