linux-kernel - Re: [PATCH 3/3] x86/speculation: Support Automatic IBRS under virtualization

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: <f25152d2-7045-94f4-d5dc-69b609c0be6a@amd.com>
Date:   Mon, 7 Nov 2022 16:29:44 -0600
From:   Kim Phillips <kim.phillips@....com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     x86@...nel.org, Borislav Petkov <bp@...en8.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Babu Moger <Babu.Moger@....com>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] x86/speculation: Support Automatic IBRS under
 virtualization

On 11/4/22 5:00 PM, Jim Mattson wrote:
> On Fri, Nov 4, 2022 at 2:38 PM Kim Phillips <kim.phillips@....com> wrote:
>>
>> VM Guests may want to use Auto IBRS, so propagate the CPUID to them.
>>
>> Co-developed-by: Babu Moger <Babu.Moger@....com>
>> Signed-off-by: Kim Phillips <kim.phillips@....com>
> 
> The APM says that, under AutoIBRS, CPL0 processes "have IBRS
> protection." I'm taking this to mean only that indirect branches in
> CPL0 are not subject to steering from a less privileged predictor
> mode. This would imply that indirect branches executed at CPL0 in L1
> could potentially be subject to steering by code running at CPL0 in
> L2, since L1 and L2 share hardware predictor modes.

That's true for AMD processors that don't support Same Mode IBRS, also
documented in the APM.

Processors that support AutoIBRS also support Same Mode IBRS (see
CPUID Fn8000_0008_EBX[IbrsSameMode] (bit 19)).

> Fortunately, there is an IBPB when switching VMCBs in svm_vcpu_load().
> But it might be worth noting that this is necessary for AutoIBRS to
> work (unless it actually isn't).

It is needed, but not for kernel/CPL0 code, rather to protect one
guest's user-space code from another's.

Kim