From: Juergen Gross <jgross@suse.com>
To: Jan Beulich <jbeulich@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: Thu, 8 Jul 2021 08:56:00 +0200 [thread overview]
Message-ID: <0fec3ba6-266d-260e-716a-ae33d7670d34@suse.com> (raw)
In-Reply-To: <fbd099f3-4d62-e0f6-1bad-6d317428051e@suse.com>
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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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next prev parent reply other threads:[~2021-07-08 6:56 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
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 [this message]
2021-05-21 10:43 ` [PATCH 0/8] xen: harden frontends against malicious backends Marek Marczykowski-Górecki
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