linux-kernel - Re: [PATCH 1/2] x86: notify hypervisor about guest entering s2idle state

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YqIJ8HtdqnoVzfQD@google.com>
Date:   Thu, 9 Jun 2022 14:55:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Grzegorz Jaszczyk <jaz@...ihalf.com>
Cc:     linux-kernel@...r.kernel.org, dmy@...ihalf.com,
        Zide Chen <zide.chen@...el.corp-partner.google.com>,
        Peter Fang <peter.fang@...el.corp-partner.google.com>,
        Tomasz Nowicki <tn@...ihalf.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, Pavel Machek <pavel@....cz>,
        Brijesh Singh <brijesh.singh@....com>,
        Ashish Kalra <ashish.kalra@....com>,
        Mario Limonciello <mario.limonciello@....com>,
        Pratik Vishwakarma <Pratik.Vishwakarma@....com>,
        Hans de Goede <hdegoede@...hat.com>,
        Sachi King <nakato@...ato.io>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        David Dunn <daviddunn@...gle.com>,
        Wei Wang <wei.w.wang@...el.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "open list:KERNEL VIRTUAL MACHINE (KVM)" <kvm@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        "open list:ACPI" <linux-acpi@...r.kernel.org>,
        "open list:HIBERNATION (aka Software Suspend, aka swsusp)" 
        <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86: notify hypervisor about guest entering s2idle
 state

On Thu, Jun 09, 2022, Grzegorz Jaszczyk wrote:
> +9. KVM_HC_SYSTEM_S2IDLE
> +------------------------
> +
> +:Architecture: x86
> +:Status: active
> +:Purpose: Notify the hypervisor that the guest is entering s2idle state.

What about exiting s2idle?  E.g.

  1. VM0 enters s2idle
  2. host notes that VM0 is in s2idle
  3. VM0 exits s2idle
  4. host still thinks VM0 is in s2idle
  5. VM1 enters s2idle
  6. host thinks all VMs are in s2idle, suspends the system

> +static void s2idle_hypervisor_notify(void)
> +{
> +	if (static_cpu_has(X86_FEATURE_HYPERVISOR))
> +		kvm_hypercall0(KVM_HC_SYSTEM_S2IDLE);

Checking the HYPERVISOR flag is not remotely sufficient.  The hypervisor may not
be KVM, and if it is KVM, it may be an older version of KVM that doesn't support
the hypercall.  The latter scenario won't be fatal unless KVM has been modified,
but blindly doing a hypercall for a different hypervisor could have disastrous
results, e.g. the registers ABIs are different, so the above will make a random
request depending on what is in other GPRs.

The bigger question is, why is KVM involved at all?  KVM is just a dumb pipe out
to userspace, and not a very good one at that.  There are multiple well established
ways to communicate with the VMM without custom hypercalls.

I bet if you're clever this can even be done without any guest changes, e.g. I
gotta imagine acpi_sleep_run_lps0_dsm() triggers MMIO/PIO with the right ACPI
configuration.