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[66.29.164.166]) by smtp.gmail.com with ESMTPSA id u2sm2096104pjv.30.2019.06.28.06.45.01 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 28 Jun 2019 06:45:01 -0700 (PDT) Subject: Re: [PATCH] block, bfq: NULL out the bic when it's no longer valid To: Douglas Anderson , Paolo Valente Cc: groeck@chromium.org, drinkcat@chromium.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20190628044409.128823-1-dianders@chromium.org> From: Jens Axboe Message-ID: <9048aa45-ecf3-c80d-a973-4aeb86adde23@kernel.dk> Date: Fri, 28 Jun 2019 07:45:00 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.1 MIME-Version: 1.0 In-Reply-To: <20190628044409.128823-1-dianders@chromium.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 List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/27/19 10:44 PM, Douglas Anderson wrote: > In reboot tests on several devices we were seeing a "use after free" > when slub_debug or KASAN was enabled. The kernel complained about: > > Unable to handle kernel paging request at virtual address 6b6b6c2b > > ...which is a classic sign of use after free under slub_debug. The > stack crawl in kgdb looked like: > > 0 test_bit (addr=, nr=) > 1 bfq_bfqq_busy (bfqq=) > 2 bfq_select_queue (bfqd=) > 3 __bfq_dispatch_request (hctx=) > 4 bfq_dispatch_request (hctx=) > 5 0xc056ef00 in blk_mq_do_dispatch_sched (hctx=0xed249440) > 6 0xc056f728 in blk_mq_sched_dispatch_requests (hctx=0xed249440) > 7 0xc0568d24 in __blk_mq_run_hw_queue (hctx=0xed249440) > 8 0xc0568d94 in blk_mq_run_work_fn (work=) > 9 0xc024c5c4 in process_one_work (worker=0xec6d4640, work=0xed249480) > 10 0xc024cff4 in worker_thread (__worker=0xec6d4640) > > Digging in kgdb, it could be found that, though bfqq looked fine, > bfqq->bic had been freed. > > Through further digging, I postulated that perhaps it is illegal to > access a "bic" (AKA an "icq") after bfq_exit_icq() had been called > because the "bic" can be freed at some point in time after this call > is made. I confirmed that there certainly were cases where the exact > crashing code path would access the "bic" after bfq_exit_icq() had > been called. Sspecifically I set the "bfqq->bic" to (void *)0x7 and > saw that the bic was 0x7 at the time of the crash. > > To understand a bit more about why this crash was fairly uncommon (I > saw it only once in a few hundred reboots), you can see that much of > the time bfq_exit_icq_fbqq() fully frees the bfqq and thus it can't > access the ->bic anymore. The only case it doesn't is if > bfq_put_queue() sees a reference still held. > > However, even in the case when bfqq isn't freed, the crash is still > rare. Why? I tracked what happened to the "bic" after the exit > routine. It doesn't get freed right away. Rather, > put_io_context_active() eventually called put_io_context() which > queued up freeing on a workqueue. The freeing then actually happened > later than that through call_rcu(). Despite all these delays, some > extra debugging showed that all the hoops could be jumped through in > time and the memory could be freed causing the original crash. Phew! > > To make a long story short, assuming it truly is illegal to access an > icq after the "exit_icq" callback is finished, this patch is needed. > > Signed-off-by: Douglas Anderson > --- > Most of the testing of this was done on the Chrome OS 4.19 kernel with > BFQ backported (thanks to Paolo's help). I did manage to reproduce a > crash on mainline Linux (v5.2-rc6) though. > > To see some of the techniques used to debug this, see > and . > > I'll also note that on linuxnext (next-20190627) I saw some other > use-after-frees that seemed related to BFQ but haven't had time to > debug. They seemed unrelated. Applied for 5.3, but I marked it for stable as well. -- Jens Axboe