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From:   Kees Cook <keescook@chromium.org>
Date:   Tue, 17 Apr 2018 09:30:59 -0700
Message-ID: <CAGXu5jJK3JuwLBGLbS1i6hBdyg_mkbVibbu6irfsnTOE1QhtfA@mail.gmail.com>
Subject: Re: usercopy whitelist woe in scsi_sense_cache
To:     James Bottomley <jejb@linux.vnet.ibm.com>
Cc:     Oleksandr Natalenko <oleksandr@natalenko.name>,
        Jens Axboe <axboe@kernel.dk>,
        Bart Van Assche <bart.vanassche@wdc.com>,
        Paolo Valente <paolo.valente@linaro.org>,
        David Windsor <dave@nullcore.net>,
        "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 3:02 AM, James Bottomley
<jejb@linux.vnet.ibm.com> wrote:
> On Mon, 2018-04-16 at 20:12 -0700, Kees Cook wrote:
>> I still haven't figured this out, though... any have a moment to look
>> at this?
>
> Just to let you know you're not alone ... but I can't make any sense of
> this either.  The bfdq is the elevator_data, which is initialised when
> the scheduler is attached, so it shouldn't change.  Is it possible to
> set a data break point on elevator_data after it's initialised and see
> if it got changed by something?

Yeah, it seems like some pointer chain is getting overwritten outside
of a lock or rcu or ?. I don't know this code well enough to guess at
where to check, though. What I find so strange is that the structure
offsets are different between bfpd's rq_in_driver field and
scsi_request's sense field, so even THAT doesn't look to be clear-cut
either:

struct bfq_data {
        struct request_queue *     queue;                /*     0     8 */
        struct list_head           dispatch;             /*     8    16 */
        struct bfq_group *         root_group;           /*    24     8 */
        struct rb_root             queue_weights_tree;   /*    32     8 */
        struct rb_root             group_weights_tree;   /*    40     8 */
        int                        busy_queues;          /*    48     4 */
        int                        wr_busy_queues;       /*    52     4 */
        int                        queued;               /*    56     4 */
        int                        rq_in_driver;         /*    60     4 */
...

struct scsi_request {
        unsigned char              __cmd[16];            /*     0    16 */
        unsigned char *            cmd;                  /*    16     8 */
        short unsigned int         cmd_len;              /*    24     2 */

        /* XXX 2 bytes hole, try to pack */

        int                        result;               /*    28     4 */
        unsigned int               sense_len;            /*    32     4 */
        unsigned int               resid_len;            /*    36     4 */
        int                        retries;              /*    40     4 */

        /* XXX 4 bytes hole, try to pack */

        void *                     sense;                /*    48     8 */
...

This is _so_ weird. :P

-Kees

-- 
Kees Cook
Pixel Security