linux-kernel - Re: [PATCH v14 00/13] Introduce support for guest CET feature

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Date:   Thu, 28 Jan 2021 10:24:55 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, jmattson@...gle.com,
        yu.c.zhang@...ux.intel.com
Subject: Re: [PATCH v14 00/13] Introduce support for guest CET feature

On Thu, Jan 28, 2021, Paolo Bonzini wrote:
> On 28/01/21 19:04, Sean Christopherson wrote:
> > On Thu, Jan 28, 2021, Paolo Bonzini wrote:
> > > On 06/11/20 02:16, Yang Weijiang wrote:
> > > > Control-flow Enforcement Technology (CET) provides protection against
> > > > Return/Jump-Oriented Programming (ROP/JOP) attack. There're two CET
> > > > sub-features: Shadow Stack (SHSTK) and Indirect Branch Tracking (IBT).
> > > > SHSTK is to prevent ROP programming and IBT is to prevent JOP programming.
> > 
> > ...
> > 
> > > I reviewed the patch and it is mostly okay.  However, if I understand it
> > > correctly, it will not do anything until host support materializes, because
> > > otherwise XSS will be 0.
> > 
> > IIRC, it won't even compile due to the X86_FEATURE_SHSTK and X86_FEATURE_IBT
> > dependencies.
> 
> Of course, but if that was the only issue I would sort it out with Boris as
> usual.  OTOH if it is dead code I won't push it to Linus.

Yes, at best it's dead code.  At worst, if it somehow became undead, the guest
state would bleed into the host and wouldn't be migrated as the kernel wouldn't
touch CET state when doing XSAVES/XRSTORS.

I floated the idea of pulling in just enough of the kernel bits to enable KVM,
but that didn't go anywhere.

https://lkml.kernel.org/r/20200723162531.GF21891@linux.intel.com