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[73.241.217.19]) by smtp.gmail.com with ESMTPSA id x21sm14195050pfc.151.2020.11.30.06.58.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Nov 2020 06:58:02 -0800 (PST) Subject: Re: [PATCH V1] block: Fix use-after-free while iterating over requests To: Hannes Reinecke , John Garry , Pradeep P V K , axboe@kernel.dk, linux-block@vger.kernel.org Cc: stummala@codeaurora.org, linux-kernel@vger.kernel.org, Ming Lei References: <1606402925-24420-1-git-send-email-ppvk@codeaurora.org> <693ea723-aa9e-1166-8a19-a7787f724969@huawei.com> <0c925db8-e481-5f21-b0fe-f691142b0437@suse.de> From: Bart Van Assche Message-ID: <5c3ac5af-ed81-11e4-fee3-f92175f14daf@acm.org> Date: Mon, 30 Nov 2020 06:58:00 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <0c925db8-e481-5f21-b0fe-f691142b0437@suse.de> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/29/20 11:04 PM, Hannes Reinecke wrote: > On 11/26/20 5:49 PM, John Garry wrote: >> On 26/11/2020 16:27, Bart Van Assche wrote: >>> On 11/26/20 7:02 AM, Pradeep P V K wrote: >>>> Observes below crash while accessing (use-after-free) request queue >>>> member of struct request. >>>> >>>> 191.784789:   <2> Unable to handle kernel paging request at virtual >>>> address ffffff81429a4440 >>>> ... >>>> 191.786174:   <2> CPU: 3 PID: 213 Comm: kworker/3:1H Tainted: G S >>>> O      5.4.61-qgki-debug-ge45de39 #1 >>>> ... >>>> 191.786226:   <2> Workqueue: kblockd blk_mq_timeout_work >>>> 191.786242:   <2> pstate: 20c00005 (nzCv daif +PAN +UAO) >>>> 191.786261:   <2> pc : bt_for_each+0x114/0x1a4 >>>> 191.786274:   <2> lr : bt_for_each+0xe0/0x1a4 >>>> ... >>>> 191.786494:   <2> Call trace: >>>> 191.786507:   <2>  bt_for_each+0x114/0x1a4 >>>> 191.786519:   <2>  blk_mq_queue_tag_busy_iter+0x60/0xd4 >>>> 191.786532:   <2>  blk_mq_timeout_work+0x54/0xe8 >>>> 191.786549:   <2>  process_one_work+0x2cc/0x568 >>>> 191.786562:   <2>  worker_thread+0x28c/0x518 >>>> 191.786577:   <2>  kthread+0x160/0x170 >>>> 191.786594:   <2>  ret_from_fork+0x10/0x18 >>>> 191.786615:   <2> Code: 0b080148 f9404929 f8685921 b4fffe01 (f9400028) >>>> 191.786630:   <2> ---[ end trace 0f1f51d79ab3f955 ]--- >>>> 191.786643:   <2> Kernel panic - not syncing: Fatal exception >>>> >>>> Fix this by updating the freed request with NULL. >>>> This could avoid accessing the already free request from other >>>> contexts while iterating over the requests. >>>> >>>> Signed-off-by: Pradeep P V K >>>> --- >>>>   block/blk-mq.c | 1 + >>>>   block/blk-mq.h | 1 + >>>>   2 files changed, 2 insertions(+) >>>> >>>> diff --git a/block/blk-mq.c b/block/blk-mq.c >>>> index 55bcee5..9996cb1 100644 >>>> --- a/block/blk-mq.c >>>> +++ b/block/blk-mq.c >>>> @@ -492,6 +492,7 @@ static void __blk_mq_free_request(struct request >>>> *rq) >>>>       blk_crypto_free_request(rq); >>>>       blk_pm_mark_last_busy(rq); >>>> +    hctx->tags->rqs[rq->tag] = NULL; >>>>       rq->mq_hctx = NULL; >>>>       if (rq->tag != BLK_MQ_NO_TAG) >>>>           blk_mq_put_tag(hctx->tags, ctx, rq->tag); >>>> diff --git a/block/blk-mq.h b/block/blk-mq.h >>>> index a52703c..8747bf1 100644 >>>> --- a/block/blk-mq.h >>>> +++ b/block/blk-mq.h >>>> @@ -224,6 +224,7 @@ static inline int >>>> __blk_mq_active_requests(struct blk_mq_hw_ctx *hctx) >>>>   static inline void __blk_mq_put_driver_tag(struct blk_mq_hw_ctx >>>> *hctx, >>>>                          struct request *rq) >>>>   { >>>> +    hctx->tags->rqs[rq->tag] = NULL; >>>>       blk_mq_put_tag(hctx->tags, rq->mq_ctx, rq->tag); >>>>       rq->tag = BLK_MQ_NO_TAG; >>> >>> Is this perhaps a block driver bug instead of a block layer core bug? If >>> this would be a block layer core bug, it would have been reported >>> before. >> >> Isn't this the same issue which as been reported many times: >> >> https://lore.kernel.org/linux-block/20200820180335.3109216-1-ming.lei@redhat.com/ >> >> >> https://lore.kernel.org/linux-block/8376443a-ec1b-0cef-8244-ed584b96fa96@huawei.com/ >> >> >> But I never saw a crash, just kasan report. >> > And if that above were a concern, I would have thought one would need to > use a WRITE_ONCE() here; otherwise we might have a race condition where > other CPUs still see the old value, no? Hi Hannes, Freeing tag->rqs and tags->static_rqs with kfree_rcu() is probably a better solution than clearing request pointers. Even when using WRITE_ONCE() to clear tag pointers, it is still possible that another thread read the tag pointer before the WRITE_ONCE() and uses it after the WRITE_ONCE() has finished. Thanks, Bart.