Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: usercopy whitelist woe in scsi_sense_cache
To:     Paolo Valente <paolo.valente@linaro.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Bart Van Assche <bart.vanassche@wdc.com>,
        David Windsor <dave@nullcore.net>,
        "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        linux-scsi@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Hannes Reinecke <hare@suse.com>,
        Johannes Thumshirn <jthumshirn@suse.de>,
        linux-block@vger.kernel.org
References: <10360653.ov98egbaqx@natalenko.name>
 <CAGXu5j+dh1zm32iBjRVh0s+3gLJYgjwt+z1aBJmrK2Cr8XPWnA@mail.gmail.com>
 <CAGXu5jLhnWnUiscP9B=kTEFNyRw9jgFn7Zr5FxhZrO20553+1Q@mail.gmail.com>
 <CAGXu5jKuZwicyyXH2gB3vp-OjtS+MkG+hqxV1cq1jaG9vAAO=g@mail.gmail.com>
 <CAGXu5jLQuzpjFcfjpT=MvgOxBizFNMjnfmY+E7eCHkDAV5swHg@mail.gmail.com>
 <CAGXu5jJTMqbXia7TmN2yEW11SM+XC3pkMt=bb+xBrY7-OOf=tw@mail.gmail.com>
 <CAGXu5j+xX3kDDS_sPY44hPVb=8isMX-evYzPx3ur=rge1rL5JQ@mail.gmail.com>
 <CAGXu5jJmvKcANUSsCz3fjGSv-tv1hceNd3rOpT4WTfYRb65RRA@mail.gmail.com>
 <8473f909-2123-0cfc-43b1-beba0b1aef9b@kernel.dk>
 <CAGXu5jL9miO3OXpjAxm9t8v=f_f5StB=Tk=aV8BVRdEj-Y-PDA@mail.gmail.com>
 <CAGXu5jKVPQi0cdTbe5Y1DNHdiJ6-kEFRBTNv7znuqT9w5eifGg@mail.gmail.com>
 <07f263ff-cea6-ac3c-944b-0f36fee8ba25@kernel.dk>
 <CAGXu5j+kHipvoyFWjGp8M6+83zbd6yHOzwxQ-2LNBQbD5-jS9w@mail.gmail.com>
 <8b32e079-d4e6-3fea-a89d-ff856e4e13b1@kernel.dk>
 <0fbf2b13-8bae-c7c5-d930-ebaafdc72202@kernel.dk>
 <011EF7D1-B095-4B8D-AD2A-993048932C49@linaro.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <c30400f9-50e6-b5cf-d06b-a5e971e98546@kernel.dk>
Date:   Wed, 18 Apr 2018 08:30:35 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.0
MIME-Version: 1.0
In-Reply-To: <011EF7D1-B095-4B8D-AD2A-993048932C49@linaro.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 4/18/18 3:08 AM, Paolo Valente wrote:
> 
> 
>> Il giorno 18 apr 2018, alle ore 00:57, Jens Axboe <axboe@kernel.dk> ha scritto:
>>
>> On 4/17/18 3:48 PM, Jens Axboe wrote:
>>> On 4/17/18 3:47 PM, Kees Cook wrote:
>>>> On Tue, Apr 17, 2018 at 2:39 PM, Jens Axboe <axboe@kernel.dk> wrote:
>>>>> On 4/17/18 3:25 PM, Kees Cook wrote:
>>>>>> On Tue, Apr 17, 2018 at 1:46 PM, Kees Cook <keescook@chromium.org> wrote:
>>>>>>> I see elv.priv[1] assignments made in a few places -- is it possible
>>>>>>> there is some kind of uninitialized-but-not-NULL state that can leak
>>>>>>> in there?
>>>>>>
>>>>>> Got it. This fixes it for me:
>>>>>>
>>>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>>>> index 0dc9e341c2a7..859df3160303 100644
>>>>>> --- a/block/blk-mq.c
>>>>>> +++ b/block/blk-mq.c
>>>>>> @@ -363,7 +363,7 @@ static struct request *blk_mq_get_request(struct
>>>>>> request_queue *q,
>>>>>>
>>>>>>        rq = blk_mq_rq_ctx_init(data, tag, op);
>>>>>>        if (!op_is_flush(op)) {
>>>>>> -               rq->elv.icq = NULL;
>>>>>> +               memset(&rq->elv, 0, sizeof(rq->elv));
>>>>>>                if (e && e->type->ops.mq.prepare_request) {
>>>>>>                        if (e->type->icq_cache && rq_ioc(bio))
>>>>>>                                blk_mq_sched_assign_ioc(rq, bio);
>>>>>> @@ -461,7 +461,7 @@ void blk_mq_free_request(struct request *rq)
>>>>>>                        e->type->ops.mq.finish_request(rq);
>>>>>>                if (rq->elv.icq) {
>>>>>>                        put_io_context(rq->elv.icq->ioc);
>>>>>> -                       rq->elv.icq = NULL;
>>>>>> +                       memset(&rq->elv, 0, sizeof(rq->elv));
>>>>>>                }
>>>>>>        }
>>>>>
>>>>> This looks like a BFQ problem, this should not be necessary. Paolo,
>>>>> you're calling your own prepare request handler from the insert
>>>>> as well, and your prepare request does nothing if rq->elv.icq == NULL.
>>>>
>>>> I sent the patch anyway, since it's kind of a robustness improvement,
>>>> I'd hope. If you fix BFQ also, please add:
>>>
>>> It's also a memset() in the hot path, would prefer to avoid that...
>>> The issue here is really the convoluted bfq usage of insert/prepare,
>>> I'm sure Paolo can take it from here.
>>
> 
> Hi,
> I'm very sorry for tuning in very late, but, at the same time, very
> glad to find the problem probably already solved ;) (in this respect, I swear,
> my delay was not intentional)
> 
>> Does this fix it?
>>
>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>> index f0ecd98509d8..d883469a1582 100644
>> --- a/block/bfq-iosched.c
>> +++ b/block/bfq-iosched.c
>> @@ -4934,8 +4934,11 @@ static void bfq_prepare_request(struct request *rq, struct bio *bio)
>> 	bool new_queue = false;
>> 	bool bfqq_already_existing = false, split = false;
>>
>> -	if (!rq->elv.icq)
>> +	if (!rq->elv.icq) {
>> +		rq->elv.priv[0] = rq->elv.priv[1] = NULL;
>> 		return;
>> +	}
>> +
> 
> This does solve the problem at hand.  But it also arouses a question,
> related to a possible subtle bug.
> 
> For BFQ, !rq->elv.icq basically means "this request is not for me, as
> I am an icq-based scheduler".  But, IIUC the main points in this
> thread, then this assumption is false.  If it is actually false, then
> I hope that all requests with !rq->elv.icq that are sent to BFQ do
> verify the condition (at_head || blk_rq_is_passthrough(rq)).  In fact,
> requests that do not verify that condition are those that BFQ must put
> in a bfq_queue.  So, even if this patch makes the crash disappear, we
> drive BFQ completely crazy (and we may expect other strange failures)
> if we send BFQ a request with !((at_head || blk_rq_is_passthrough(rq))
> and !rq->elv.icq.  BFQ has to put that rq into a bfq_queue, but simply
> cannot.
> 
> Jens, or any other, could you please shed a light on this, and explain
> how things are exactly?

Your assumption is correct, however you set ->priv[0] and ->priv[1] for
requests, but only for ->elv.icq != NULL. So let's assume you get a
request and assign those two, request completes. Later on, you get
the same request, bypass insert it. BFQ doesn't clear the bic/bfqq
pointers in the request, since ->elv.icq == NULL. It gets inserted
into the dispatch list.

Then when __bfq_dispatch_request() is called, you do:

bfqq = RQ_BFQQ(rq);
if (bfqq)
	bfqq->dispatched++;
[...]

which is wrong, since you don't know if you assigned a bfqq for this
request. The memory that bfqq points to could be long gone, if that
queue is freed. So you could either guard any bfqq/bic retrieval
with ->elv.icq != NULL, or you could just clear the pointers for
the case where the values aren't valid.

-- 
Jens Axboe