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

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Date:   Thu, 8 Jul 2021 08:56:00 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Roger Pau Monné <roger.pau@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, xen-devel@...ts.xenproject.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data
 blindly

On 08.07.21 08:52, Jan Beulich wrote:
> On 08.07.2021 08:40, Juergen Gross wrote:
>> On 08.07.21 08:37, Jan Beulich wrote:
>>> On 08.07.2021 07:47, Juergen Gross wrote:
>>>> On 17.05.21 17:33, Jan Beulich wrote:
>>>>> 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.
>>>>
>>>> Oh, now I get your problem.
>>>>
>>>> But shouldn't that better be solved by using READ_ONCE() for reading rp
>>>> instead?
>>>
>>> Not sure - the rmb() is needed anyway aiui, and hence you could as well
>>> move your code addition.
>>
>> Sure.
>>
>> My question was rather: does the rmb() really eliminate the possibility
>> of a double read introduced by the compiler? If yes, moving the code is
>> the correct solution.
> 
> It doesn't eliminate the possibility of a double read, but (leaving
> aside split accesses) that's not what you care about here. What you
> need is a single stable value to operate on. No matter how many
> (non-split) reads the compiler may issue to fill "rp", the final
> read's value will be used in the subsequent calculation. Or at
> least that's been my understanding; thinking about it the compiler
> might issue multiple reads into distinct registers ahead of the
> barrier, and use different registers for different subsequent
> operations. While this would look like intentionally inefficient
> code generation to me, you may indeed want to play safe and use
> ACCESS_ONCE() _and_ the barrier. I guess there are more places then
> which would want similar treatment, and it's not a problem that
> this change introduces ...

Nevertheless I think I can change it right away. It will also help
against load tearing.


Juergen

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