linux-kernel - Re: [PATCH v10 00/26] Control-flow Enforcement: Shadow Stack

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Date:   Thu, 23 Jul 2020 09:41:37 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v10 00/26] Control-flow Enforcement: Shadow Stack

On 7/23/20 9:25 AM, Sean Christopherson wrote:
> How would people feel about taking the above two patches (02 and 03 in the
> series) through the KVM tree to enable KVM virtualization of CET before the
> kernel itself gains CET support?  I.e. add the MSR and feature bits, along
> with the XSAVES context switching.  The feature definitons could use "" to
> suppress displaying them in /proc/cpuinfo to avoid falsely advertising CET
> to userspace.
> 
> AIUI, there are ABI issues that need to be sorted out, and that is likely
> going to drag on for some time. 
> 
> Is this a "hell no" sort of idea, or something that would be feasible if we
> can show that there are no negative impacts to the kernel?

Negative impacts like bloating every task->fpu with XSAVE state that
will never get used? ;)

I thought KVM had its own vcpu->arch.guest_fpu buffers which mirrored
the size and format of task->fpu.  Can we have KVM support today without
task->fpu support?  I see some XSS munging in the KVM code so I think
this might be *possible*, but I don't see all of the plumbing that would
make it actually work.