Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data blindly

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.05.2021 17:22, Juergen Gross wrote:
> On 17.05.21 17:12, Jan Beulich wrote:
>> On 17.05.2021 16:23, Juergen Gross wrote:
>>> On 17.05.21 16:11, Jan Beulich wrote:
>>>> On 13.05.2021 12:02, Juergen Gross wrote:
>>>>> @@ -1574,10 +1580,16 @@ static irqreturn_t blkif_interrupt(int irq, void 
>>>>> *dev_id)
>>>>>           spin_lock_irqsave(&rinfo->ring_lock, flags);
>>>>>     again:
>>>>>           rp = rinfo->ring.sring->rsp_prod;
>>>>> + if (RING_RESPONSE_PROD_OVERFLOW(&rinfo->ring, rp)) {
>>>>> +         pr_alert("%s: illegal number of responses %u\n",
>>>>> +                  info->gd->disk_name, rp - rinfo->ring.rsp_cons);
>>>>> +         goto err;
>>>>> + }
>>>>>           rmb(); /* Ensure we see queued responses up to 'rp'. */
>>>>
>>>> I think you want to insert after the barrier.
>>>
>>> Why? The relevant variable which is checked is "rp". The result of the
>>> check is in no way depending on the responses themselves. And any change
>>> of rsp_cons is protected by ring_lock, so there is no possibility of
>>> reading an old value here.
>>
>> But this is a standard double read situation: You might check a value
>> and then (via a separate read) use a different one past the barrier.
> 
> Yes and no.
> 
> rsp_cons should never be written by the other side, and additionally
> it would be read multiple times anyway.

But I'm talking about rsp_prod, as that's what rp gets loaded from.

Jan

> So if the other side is writing it, the write could always happen after
> the test and before the loop is started. This is no real issue here as
> the frontend would very soon stumble over an illegal response (either
> no request pending, or some other inconsistency). The test is meant to
> have a more detailed error message in case it hits.
> 
> In the end it doesn't really matter, so I can change it. I just wanted
> to point out that IMO both variants are equally valid.
> 
> 
> Juergen
>