From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5BD8EC433F5 for ; Mon, 28 Feb 2022 14:30:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237183AbiB1ObK (ORCPT ); Mon, 28 Feb 2022 09:31:10 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43666 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231190AbiB1ObG (ORCPT ); Mon, 28 Feb 2022 09:31:06 -0500 Received: from mail-oi1-x231.google.com (mail-oi1-x231.google.com [IPv6:2607:f8b0:4864:20::231]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 750B97EB33 for ; Mon, 28 Feb 2022 06:30:27 -0800 (PST) Received: by mail-oi1-x231.google.com with SMTP id i5so13350732oih.1 for ; Mon, 28 Feb 2022 06:30:27 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=FRLe3AnC5pgmPAqdinsXHEKECzNJf8GOWiq9+hi8EA0=; b=O6D5YigaeV9BY25z8CJpwRAv0C2AKw5rbsNPY68LMHf19POHkcqi8mqIg0L5XVgCvn Ff89gUNxifobG2fV1RDPUskHLpXQHXgOexocbpm1GyqM2EALbtVkQElWlKrXsBTS9TjU XYdpXvjFPQXQ2kdb30tQhCaZ6jTzMfG6uZswisN8wCS0sWSJ3Qxe6vTgy+zPqX4vLRdn vtqM7GQu1SaPK7HO0yidE3WbIqtCSMGUWcvo7ZmgQs+p5DW7Ssk8VaiyhlNP1DPNdtY+ VnECCWTwwtJZq/bPywWnc1f6nQoLZM7lGOmkMgpWWgmpSHYQxK/isnSvjw/lrJv34XqZ 0Vsg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=FRLe3AnC5pgmPAqdinsXHEKECzNJf8GOWiq9+hi8EA0=; b=zVBBLm5AyR6UK2IoqDvQPau+QIENDAUFwbXwRKYR7zQmG//Hh4dquaAPRhYHVEsJN7 jW+5yWc8Kefw+9p0nO/WBkePb7QcxPO8C6Uq4tkzj2W/DdFt14KbDJidlNGtzxmL8DWG OPdKe7rVUYRhMZJM+j0ajRwLKSgWRg4spjecMScTQE2TjOTW67Qh94xUeqb8v7StMU1M tm9kNSzwbH4Om+OGYHwgSzCNtqGYu7cq6fY/DhiKTvzp6cxEuQMESQ2tP8j+5F3MfjAK 5eCEZ4dPneCqT0ZMqMElPhPlS75a2fQ29dGjQslgI4SKHuC/o7UsaOMnWV7+d13pkhVZ ix3w== X-Gm-Message-State: AOAM531uCRCgeZ9+YvvGt8GHoHvi6TiA5y4VHbd07/R6s8CvoooPgSSE qG58DBm23GpfeUwwr2AAE2zWTMGbpnH2rcCyuLx2Fw== X-Google-Smtp-Source: ABdhPJyE1Bo4A6f0RU7lZ+Mwlo5pbLsjN6HBPgLuRnOq795Tp7+xm/lq2BC9S/szFYWPn1VKoattLvYoKlO70i+4/98= X-Received: by 2002:a05:6808:1999:b0:2d6:7fe3:10bd with SMTP id bj25-20020a056808199900b002d67fe310bdmr10721866oib.68.1646058626511; Mon, 28 Feb 2022 06:30:26 -0800 (PST) MIME-Version: 1.0 References: <20220223062412.22334-1-chenyi.qiang@intel.com> <88eb9a9a-fbe3-8e2c-02bd-4bdfc855b67f@intel.com> <6a839b88-392d-886d-836d-ca04cf700dce@intel.com> <7859e03f-10fa-dbc2-ed3c-5c09e62f9016@redhat.com> <71736b9d-9ed4-ea02-e702-74cae0340d66@intel.com> <4b2ddc09-f68d-1cc3-3d10-f7651d811fc3@intel.com> In-Reply-To: <4b2ddc09-f68d-1cc3-3d10-f7651d811fc3@intel.com> From: Jim Mattson Date: Mon, 28 Feb 2022 06:30:15 -0800 Message-ID: Subject: Re: [PATCH v3] KVM: VMX: Enable Notify VM exit To: Xiaoyao Li Cc: Paolo Bonzini , Chenyi Qiang , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Feb 27, 2022 at 11:10 PM Xiaoyao Li wrote: > > On 2/26/2022 10:24 PM, Jim Mattson wrote: > > On Fri, Feb 25, 2022 at 10:24 PM Xiaoyao Li wrote: > >> > >> On 2/26/2022 12:53 PM, Jim Mattson wrote: > >>> On Fri, Feb 25, 2022 at 8:25 PM Jim Mattson wrote: > >>>> > >>>> On Fri, Feb 25, 2022 at 8:07 PM Xiaoyao Li wrote: > >>>>> > >>>>> On 2/25/2022 11:13 PM, Paolo Bonzini wrote: > >>>>>> On 2/25/22 16:12, Xiaoyao Li wrote: > >>>>>>>>>> > >>>>>>>>> > >>>>>>>>> I don't like the idea of making things up without notifying userspace > >>>>>>>>> that this is fictional. How is my customer running nested VMs supposed > >>>>>>>>> to know that L2 didn't actually shutdown, but L0 killed it because the > >>>>>>>>> notify window was exceeded? If this information isn't reported to > >>>>>>>>> userspace, I have no way of getting the information to the customer. > >>>>>>>> > >>>>>>>> Then, maybe a dedicated software define VM exit for it instead of > >>>>>>>> reusing triple fault? > >>>>>>>> > >>>>>>> > >>>>>>> Second thought, we can even just return Notify VM exit to L1 to tell > >>>>>>> L2 causes Notify VM exit, even thought Notify VM exit is not exposed > >>>>>>> to L1. > >>>>>> > >>>>>> That might cause NULL pointer dereferences or other nasty occurrences. > >>>>> > >>>>> IMO, a well written VMM (in L1) should handle it correctly. > >>>>> > >>>>> L0 KVM reports no Notify VM Exit support to L1, so L1 runs without > >>>>> setting Notify VM exit. If a L2 causes notify_vm_exit with > >>>>> invalid_vm_context, L0 just reflects it to L1. In L1's view, there is no > >>>>> support of Notify VM Exit from VMX MSR capability. Following L1 handler > >>>>> is possible: > >>>>> > >>>>> a) if (notify_vm_exit available & notify_vm_exit enabled) { > >>>>> handle in b) > >>>>> } else { > >>>>> report unexpected vm exit reason to userspace; > >>>>> } > >>>>> > >>>>> b) similar handler like we implement in KVM: > >>>>> if (!vm_context_invalid) > >>>>> re-enter guest; > >>>>> else > >>>>> report to userspace; > >>>>> > >>>>> c) no Notify VM Exit related code (e.g. old KVM), it's treated as > >>>>> unsupported exit reason > >>>>> > >>>>> As long as it belongs to any case above, I think L1 can handle it > >>>>> correctly. Any nasty occurrence should be caused by incorrect handler in > >>>>> L1 VMM, in my opinion. > >>>> > >>>> Please test some common hypervisors (e.g. ESXi and Hyper-V). > >>> > >>> I took a look at KVM in Linux v4.9 (one of our more popular guests), > >>> and it will not handle this case well: > >>> > >>> if (exit_reason < kvm_vmx_max_exit_handlers > >>> && kvm_vmx_exit_handlers[exit_reason]) > >>> return kvm_vmx_exit_handlers[exit_reason](vcpu); > >>> else { > >>> WARN_ONCE(1, "vmx: unexpected exit reason 0x%x\n", exit_reason); > >>> kvm_queue_exception(vcpu, UD_VECTOR); > >>> return 1; > >>> } > >>> > >>> At least there's an L1 kernel log message for the first unexpected > >>> NOTIFY VM-exit, but after that, there is silence. Just a completely > >>> inexplicable #UD in L2, assuming that L2 is resumable at this point. > >> > >> At least there is a message to tell L1 a notify VM exit is triggered in > >> L2. Yes, the inexplicable #UD won't be hit unless L2 triggers Notify VM > >> exit with invalid_context, which is malicious to L0 and L1. > > > > There is only an L1 kernel log message *the first time*. That's not > > good enough. And this is just one of the myriad of possible L1 > > hypervisors. > > > >> If we use triple_fault (i.e., shutdown), then no info to tell L1 that > >> it's caused by Notify VM exit with invalid context. Triple fault needs > >> to be extended and L1 kernel needs to be enlightened. It doesn't help > >> old guest kernel. > >> > >> If we use Machine Check, it's somewhat same inexplicable to L2 unless > >> it's enlightened. But it doesn't help old guest kernel. > >> > >> Anyway, for Notify VM exit with invalid context from L2, I don't see a > >> good solution to tell L1 VMM it's a "Notify VM exit with invalid context > >> from L2" and keep all kinds of L1 VMM happy, especially for those with > >> old kernel versions. > > > > I agree that there is no way to make every conceivable L1 happy. > > That's why the information needs to be surfaced to the L0 userspace. I > > contend that any time L0 kvm violates the architectural specification > > in its emulation of L1 or L2, the L0 userspace *must* be informed. > > We can make the design to exit to userspace on notify vm exit > unconditionally with exit_qualification passed, then userspace can take > the same action like what this patch does in KVM that > > - re-enter guest when context_invalid is false; > - stop running the guest if context_invalid is true; (userspace can > definitely re-enter the guest in this case, but it needs to take the > fall on this) > > Then, for nested case, L0 needs to enable it transparently for L2 if > this feature is enabled for L1 guest (the reason as we all agreed that > cannot allow L1 to escape just by creating a L2). Then what should KVM > do when notify vm exit from L2? > > - Exit to L0 userspace on L2's notify vm exit. L0 userspace takes the > same action: > - re-enter if context-invalid is false; > - kill L1 if context-invalid is true; (I don't know if there is any > interface for L0 userspace to kill L2). Then it opens the potential door > for malicious user to kill L1 by creating a L2 to trigger fatal notify > vm exit. If you guys accept it, we can implement in this way. > > > in conclusion, we have below solution: > > 1. Take this patch as is. The drawback is L1 VMM receives a triple_fault > from L2 when L2 triggers notify vm exit with invalid context. Neither of > L1 VMM, L1 userspace, nor L2 kernel know it's caused due to notify vm > exit. There is only kernel log in L0, which seems not accessible for L1 > user or L2 guest. You are correct on that last point, and I feel that I cannot stress it enough. In a typical environment, the L0 kernel log is only available to the administrator of the L0 host. > 2. a) Inject notify vm exit back to L1 if L2 triggers notify vm exit > with invalid context. The drawback is, old L1 hypervisor is not > enlightened of it and maybe misbehave on it. > > b) Inject a synthesized SHUTDOWN exit to L1, with additional info to > tell it's caused by fatal notify vm exit from L2. It has the same > drawback that old hypervisor has no idea of it and maybe misbehave on it. > > 3. Exit to L0 usersapce unconditionally no matter it's caused from L1 or > L2. Then it may open the door for L1 user to kill L1. > > Do you have any better solution other than above? If no, we need to pick > one from above though it cannot make everyone happy. Yes, I believe I have a better solution. We obviously need an API for userspace to synthesize a SHUTDOWN event for a vCPU. In addition, to avoid breaking legacy userspace, the NOTIFY VM-exit should be opt-in.