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

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Message-ID: <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com>
Date:   Mon, 17 May 2021 17:22:34 +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 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.

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

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