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[209.132.183.28]) by mx.google.com with ESMTPS id h71si27003602qke.354.2019.07.22.21.01.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 22 Jul 2019 21:01:59 -0700 (PDT) Received-SPF: pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jasowang@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9871C308FBB1; Tue, 23 Jul 2019 04:01:58 +0000 (UTC) Received: from [10.72.12.57] (ovpn-12-57.pek2.redhat.com [10.72.12.57]) by smtp.corp.redhat.com (Postfix) with ESMTP id D8ABF60BEC; Tue, 23 Jul 2019 04:01:42 +0000 (UTC) Subject: Re: WARNING in __mmdrop To: "Michael S. Tsirkin" Cc: syzbot , aarcange@redhat.com, akpm@linux-foundation.org, christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com, elena.reshetova@intel.com, guro@fb.com, hch@infradead.org, james.bottomley@hansenpartnership.com, jglisse@redhat.com, keescook@chromium.org, ldv@altlinux.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-parisc@vger.kernel.org, luto@amacapital.net, mhocko@suse.com, mingo@kernel.org, namit@vmware.com, peterz@infradead.org, syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk, wad@chromium.org References: <0000000000008dd6bb058e006938@google.com> <000000000000964b0d058e1a0483@google.com> <20190721044615-mutt-send-email-mst@kernel.org> <20190721081447-mutt-send-email-mst@kernel.org> <85dd00e2-37a6-72b7-5d5a-8bf46a3526cf@redhat.com> <20190722040230-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <4bd2ff78-6871-55f2-44dc-0982ffef3337@redhat.com> Date: Tue, 23 Jul 2019 12:01:40 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190722040230-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Tue, 23 Jul 2019 04:01:59 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2019/7/22 下午4:08, Michael S. Tsirkin wrote: > On Mon, Jul 22, 2019 at 01:24:24PM +0800, Jason Wang wrote: >> On 2019/7/21 下午8:18, Michael S. Tsirkin wrote: >>> On Sun, Jul 21, 2019 at 06:02:52AM -0400, Michael S. Tsirkin wrote: >>>> On Sat, Jul 20, 2019 at 03:08:00AM -0700, syzbot wrote: >>>>> syzbot has bisected this bug to: >>>>> >>>>> commit 7f466032dc9e5a61217f22ea34b2df932786bbfc >>>>> Author: Jason Wang >>>>> Date: Fri May 24 08:12:18 2019 +0000 >>>>> >>>>> vhost: access vq metadata through kernel virtual address >>>>> >>>>> bisection log:https://syzkaller.appspot.com/x/bisect.txt?x=149a8a20600000 >>>>> start commit: 6d21a41b Add linux-next specific files for 20190718 >>>>> git tree: linux-next >>>>> final crash:https://syzkaller.appspot.com/x/report.txt?x=169a8a20600000 >>>>> console output:https://syzkaller.appspot.com/x/log.txt?x=129a8a20600000 >>>>> kernel config:https://syzkaller.appspot.com/x/.config?x=3430a151e1452331 >>>>> dashboard link:https://syzkaller.appspot.com/bug?extid=e58112d71f77113ddb7b >>>>> syz repro:https://syzkaller.appspot.com/x/repro.syz?x=10139e68600000 >>>>> >>>>> Reported-by:syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com >>>>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual >>>>> address") >>>>> >>>>> For information about bisection process see:https://goo.gl/tpsmEJ#bisection >>>> OK I poked at this for a bit, I see several things that >>>> we need to fix, though I'm not yet sure it's the reason for >>>> the failures: >>>> >>>> >>>> 1. mmu_notifier_register shouldn't be called from vhost_vring_set_num_addr >>>> That's just a bad hack, in particular I don't think device >>>> mutex is taken and so poking at two VQs will corrupt >>>> memory. >>>> So what to do? How about a per vq notifier? >>>> Of course we also have synchronize_rcu >>>> in the notifier which is slow and is now going to be called twice. >>>> I think call_rcu would be more appropriate here. >>>> We then need rcu_barrier on module unload. >>>> OTOH if we make pages linear with map then we are good >>>> with kfree_rcu which is even nicer. >>>> >>>> 2. Doesn't map leak after vhost_map_unprefetch? >>>> And why does it poke at contents of the map? >>>> No one should use it right? >>>> >>>> 3. notifier unregister happens last in vhost_dev_cleanup, >>>> but register happens first. This looks wrong to me. >>>> >>>> 4. OK so we use the invalidate count to try and detect that >>>> some invalidate is in progress. >>>> I am not 100% sure why do we care. >>>> Assuming we do, uaddr can change between start and end >>>> and then the counter can get negative, or generally >>>> out of sync. >>>> >>>> So what to do about all this? >>>> I am inclined to say let's just drop the uaddr optimization >>>> for now. E.g. kvm invalidates unconditionally. >>>> 3 should be fixed independently. >>> Above implements this but is only build-tested. >>> Jason, pls take a look. If you like the approach feel >>> free to take it from here. >>> >>> One thing the below does not have is any kind of rate-limiting. >>> Given it's so easy to restart I'm thinking it makes sense >>> to add a generic infrastructure for this. >>> Can be a separate patch I guess. >> >> I don't get why must use kfree_rcu() instead of synchronize_rcu() here. > synchronize_rcu has very high latency on busy systems. > It is not something that should be used on a syscall path. > KVM had to switch to SRCU to keep it sane. > Otherwise one guest can trivially slow down another one. I think you mean the synchronize_rcu_expedited()? Rethink of the code, the synchronize_rcu() in ioctl() could be removed, since it was serialized with memory accessor. Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(), (just a little bit more hard to trigger):     case KVM_RUN: { ...         if (unlikely(oldpid != task_pid(current))) {             /* The thread running this VCPU changed. */             struct pid *newpid;             r = kvm_arch_vcpu_run_pid_change(vcpu);             if (r)                 break;             newpid = get_task_pid(current, PIDTYPE_PID);             rcu_assign_pointer(vcpu->pid, newpid);             if (oldpid)                 synchronize_rcu();             put_pid(oldpid);         } ...         break; > >>> Signed-off-by: Michael S. Tsirkin >> >> Let me try to figure out the root cause then decide whether or not to go for >> this way. >> >> Thanks > The root cause of the crash is relevant, but we still need > to fix issues 1-4. > > More issues (my patch tries to fix them too): > > 5. page not dirtied when mappings are torn down outside > of invalidate callback Yes. > > 6. potential cross-VM DOS by one guest keeping system busy > and increasing synchronize_rcu latency to the point where > another guest stars timing out and crashes > > > This will be addressed after I remove the synchronize_rcu() from ioctl path. Thanks