Re: [PATCH v2 2/3] xenfb: move xen_rmb to the correct location

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 12/04/16 13:57, David Vrabel wrote:
> On 12/04/16 11:43, Wei Liu wrote:
>> It should be placed before first time producer and consumer are used.
> This change isn't necessary and is confusing as this is not what this
> barrier is for.
>
> The barrier needs to be between the load of prod and the load of the
> ring contents (there's even a comment that says this).  This pairs with
> the corresponding write barrier between the store of the ring contents
> and the store of prod (in the other end).

Looking further, this code will compile to multiple reads of the page,
because there is no ACCESS_ONCE().  This code is still vulnerable to
XSA-155.

~Andrew

>
> David
>
>> Signed-off-by: Wei Liu <wei.liu2@citrix.com>
>> ---
>> Cc: Stefano Stabellini <sstabellini@kernel.org>
>> Cc: Anthony Perard <anthony.perard@citrix.com>
>>
>> Backport candidate to our own tree.
>> ---
>>  hw/display/xenfb.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
>> index 9866dfd..7f4fad7 100644
>> --- a/hw/display/xenfb.c
>> +++ b/hw/display/xenfb.c
>> @@ -775,10 +775,10 @@ static void xenfb_handle_events(struct XenFB *xenfb)
>>  
>>      prod = page->out_prod;
>>      out_cons = page->out_cons;
>> +    xen_rmb();
>>      if (prod - out_cons > XENFB_OUT_RING_LEN) {
>>          return;
>>      }
>> -    xen_rmb();		/* ensure we see ring contents up to prod */
>>      for (cons = out_cons; cons != prod; cons++) {
>>  	union xenfb_out_event *event = &XENFB_OUT_RING_REF(page, cons);
>>          uint8_t type = event->type;
>>
>
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