linux-kernel - Re: [PATCH 2/4] KVM: x86: Introduce paravirt feature CR0/CR4 pinning

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Date:   Thu, 9 Jul 2020 16:37:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Andersen, John" <john.s.andersen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Shuah Khan <shuah@...nel.org>,
        Liran Alon <liran.alon@...cle.com>,
        Andrew Jones <drjones@...hat.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Oliver Neukum <oneukum@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Fenghua Yu <fenghua.yu@...el.com>, reinette.chatre@...el.com,
        vineela.tummalapalli@...el.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        caoj.fnst@...fujitsu.com, Baoquan He <bhe@...hat.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Dan Williams <dan.j.williams@...el.com>, eric.auger@...hat.com,
        aaronlewis@...gle.com, Peter Xu <peterx@...hat.com>,
        makarandsonare@...gle.com,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 2/4] KVM: x86: Introduce paravirt feature CR0/CR4 pinning

On Thu, Jul 09, 2020 at 09:22:09AM -0700, Dave Hansen wrote:
> On 7/9/20 9:07 AM, Andy Lutomirski wrote:
> > On Thu, Jul 9, 2020 at 8:56 AM Dave Hansen <dave.hansen@...el.com> wrote:
> >> On 7/9/20 8:44 AM, Andersen, John wrote:
> >>>         Bits which are allowed to be pinned default to WP for CR0 and SMEP,
> >>>         SMAP, and UMIP for CR4.
> >> I think it also makes sense to have FSGSBASE in this set.
> >>
> >> I know it hasn't been tested, but I think we should do the legwork to
> >> test it.  If not in this set, can we agree that it's a logical next step?
> > I have no objection to pinning FSGSBASE, but is there a clear
> > description of the threat model that this whole series is meant to
> > address?  The idea is to provide a degree of protection against an
> > attacker who is able to convince a guest kernel to write something
> > inappropriate to CR4, right?  How realistic is this?
> 
> If a quick search can find this:
> 
> > https://googleprojectzero.blogspot.com/2017/05/exploiting-linux-kernel-via-packet.html
> 
> I'd pretty confident that the guys doing actual bad things have it in
> their toolbox too.

Right, it's common (see my commit log in 873d50d58f67), and having this
enforced by the hypervisor is WAY better since it'll block gadgets or
ROP.

-- 
Kees Cook