linux-kernel - Re: [PATCH] blk-mq: Clear out elevator private data

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Date:   Wed, 18 Apr 2018 10:47:17 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bart Van Assche <bart.vanassche@....com>,
        David Windsor <dave@...lcore.net>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Hannes Reinecke <hare@...e.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        linux-block@...r.kernel.org
Subject: Re: [PATCH] blk-mq: Clear out elevator private data



> Il giorno 17 apr 2018, alle ore 23:42, Kees Cook <keescook@...omium.org> ha scritto:
> 
> Some elevators may not correctly check rq->rq_flags & RQF_ELVPRIV, and
> may attempt to read rq->elv fields. When requests got reused, this
> caused BFQ to think it already had a bfqq (rq->elv.priv[1]) allocated.

Hi Kees,
where does BFQ gets confused and operates on a request not destined to
it?  I'm asking because I paid attention to always avoid such a
mistake.

Thanks,
Paolo

> This could lead to odd behaviors like having the sense buffer address
> slowly start incrementing. This eventually tripped HARDENED_USERCOPY
> and KASAN.
> 
> This patch wipes all of rq->elv instead of just rq->elv.icq. While
> it shouldn't technically be needed, this ends up being a robustness
> improvement that should lead to at least finding bugs in elevators faster.
> 
> Reported-by: Oleksandr Natalenko <oleksandr@...alenko.name>
> Fixes: bd166ef183c26 ("blk-mq-sched: add framework for MQ capable IO schedulers")
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> In theory, BFQ needs to also check the RQF_ELVPRIV flag, but I'll leave that
> to Paolo to figure out. Also, my Fixes line is kind of a best-guess. This
> is where icq was originally wiped, so it seemed as good a commit as any.
> ---
> block/blk-mq.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> 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));
> 		}
> 	}
> 
> -- 
> 2.7.4
> 
> 
> -- 
> Kees Cook
> Pixel Security