linux-kernel - Re: [PATCH 4/4] X86: Use KVM CR pin MSRs

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Date:   Sat, 4 Jul 2020 11:11:21 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     "Andersen, John" <john.s.andersen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        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>, mchehab+huawei@...nel.org,
        Greg KH <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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>,
        Kees Cook <keescook@...omium.org>,
        Geremy Condra <geremy.condra@...el.com>,
        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 4/4] X86: Use KVM CR pin MSRs

On Fri, Jul 03, 2020 at 09:48:14PM +0000, Andersen, John wrote:
> > > Is there a plan for fixing this for real?  I'm wondering if there is a
> > > sane weakening of this feature that still allows things like kexec.
> > > 
> > 
> > I'm pretty sure kexec can be fixed. I had it working at one point, I'm
> > currently in the process of revalidating this. The issue was though that
> > kexec only worked within the guest, not on the physical host, which I suspect
> > is related to the need for supervisor pages to be mapped, which seems to be
> > required before enabling SMAP (based on what I'd seen with the selftests and
> > unittests). I was also just blindly turning on the bits without checking for
> > support when I'd tried this, so that could have been the issue too.
> > 
> > I think most of the changes for just blindly enabling the bits were in
> > relocate_kernel, secondary_startup_64, and startup_32.
> > 
> 
> So I have a naive fix for kexec which has only been tested to work under KVM.
> When tested on a physical host, it did not boot when SMAP or UMIP were set.
> Undoubtedly it's not the correct way to do this, as it skips CPU feature
> identification, opting instead for blindly setting the bits. The physical host
> I tested this on does not have UMIP so that's likely why it failed to boot when
> UMIP gets set blindly. Within kvm-unit-tests, the test for SMAP maps memory as
> supervisor pages before enabling SMAP. I suspect this is why setting SMAP
> blindly causes the physical host not to boot.
> 
> Within trampoline_32bit_src() if I add more instructions I get an error
> about "attempt to move .org backwards", which as I understand it means
> there are only so many instructions allowed in each of those functions.
> 
> My suspicion is that someone with more knowledge of this area has a good
> idea on how best to handle this. Feedback would be much appreciated.

You can simply increase the value of TRAMPOLINE_32BIT_CODE_SIZE in
pgtable.h, assuming you don't need a very large increase. There's one
page available for code + stack at present.