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From:   Kees Cook <keescook@chromium.org>
Date:   Tue, 17 Apr 2018 13:46:04 -0700
Message-ID: <CAGXu5jL9miO3OXpjAxm9t8v=f_f5StB=Tk=aV8BVRdEj-Y-PDA@mail.gmail.com>
Subject: Re: usercopy whitelist woe in scsi_sense_cache
To:     Jens Axboe <axboe@kernel.dk>,
        Paolo Valente <paolo.valente@linaro.org>
Cc:     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
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On Tue, Apr 17, 2018 at 1:28 PM, Jens Axboe <axboe@kernel.dk> wrote:
> It has to be the latter bfqq->dispatched increment, as those are
> transient (and bfqd is not).

Yeah, and I see a lot of comments around the lifetime of rq and bfqq,
so I assume something is not being locked correctly.

#define RQ_BFQQ(rq)             ((rq)->elv.priv[1])

static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
{
        struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
        struct request *rq = NULL;
        struct bfq_queue *bfqq = NULL;

        if (!list_empty(&bfqd->dispatch)) {
                rq = list_first_entry(&bfqd->dispatch, struct request,
                                      queuelist);
                list_del_init(&rq->queuelist);

                bfqq = RQ_BFQQ(rq);

                if (bfqq) {
                        /*
                         * Increment counters here, because this
                         * dispatch does not follow the standard
                         * dispatch flow (where counters are
                         * incremented)
                         */
                        bfqq->dispatched++;
...

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?

bfq_prepare_request() assigns elv.priv[1], and bfq_insert_request()
only checks that it's non-NULL (if at all) in one case. Can
bfq_insert_request() get called without bfq_prepare_request() being
called first?

-Kees

-- 
Kees Cook
Pixel Security