From: Jan Beulich <jbeulich@suse.com>
To: Juergen Gross <jgross@suse.com>
Cc: "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
"Roger Pau Monné" <roger.pau@citrix.com>,
"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
"Stefano Stabellini" <sstabellini@kernel.org>,
"Jens Axboe" <axboe@kernel.dk>,
xen-devel@lists.xenproject.org, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data blindly
Date: Mon, 17 May 2021 17:33:42 +0200 [thread overview]
Message-ID: <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com> (raw)
In-Reply-To: <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com>
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
>
next prev parent reply other threads:[~2021-05-17 15:35 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-13 10:02 [PATCH 0/8] xen: harden frontends against malicious backends Juergen Gross
2021-05-13 10:02 ` [PATCH 2/8] xen/blkfront: read response from backend only once Juergen Gross
2021-05-17 13:50 ` Jan Beulich
2021-05-13 10:02 ` [PATCH 3/8] xen/blkfront: don't take local copy of a request from the ring page Juergen Gross
2021-05-17 14:01 ` Jan Beulich
2021-05-17 14:11 ` Juergen Gross
2021-05-13 10:02 ` [PATCH 4/8] xen/blkfront: don't trust the backend response data blindly Juergen Gross
2021-05-17 14:11 ` Jan Beulich
2021-05-17 14:23 ` Juergen Gross
2021-05-17 15:12 ` Jan Beulich
2021-05-17 15:22 ` Juergen Gross
2021-05-17 15:33 ` Jan Beulich [this message]
2021-07-08 5:47 ` Juergen Gross
2021-07-08 6:37 ` Jan Beulich
2021-07-08 6:40 ` Juergen Gross
2021-07-08 6:52 ` Jan Beulich
2021-07-08 6:56 ` Juergen Gross
2021-05-21 10:43 ` [PATCH 0/8] xen: harden frontends against malicious backends Marek Marczykowski-Górecki
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