Re: [PATCH v3 00/21] Enable CET Virtualization

[Sean Christopherson <seanjc@google.com>]

On Mon, Jun 19, 2023, Weijiang Yang wrote:
> 
> On 6/17/2023 1:56 AM, Sean Christopherson wrote:
> > On Fri, Jun 16, 2023, Weijiang Yang wrote:
> > > On 6/16/2023 7:30 AM, Sean Christopherson wrote:
> > > > On Thu, May 11, 2023, Yang Weijiang wrote:
> > > > > The last patch is introduced to support supervisor SHSTK but the feature is
> > > > > not enabled on Intel platform for now, the main purpose of this patch is to
> > > > > facilitate AMD folks to enable the feature.
> > > > I am beyond confused by the SDM's wording of CET_SSS.
> > > > 
> > > > First, it says that CET_SSS says the CPU isn't buggy (or maybe "less buggy" is
> > > > more appropriate phrasing).
> > > > 
> > > >     Bit 18: CET_SSS. If 1, indicates that an operating system can enable supervisor
> > > >     shadow stacks as long as it ensures that certain supervisor shadow-stack pushes
> > > >     will not cause page faults (see Section 17.2.3 of the Intel® 64 and IA-32
> > > >     Architectures Software Developer’s Manual, Volume 1).
> > > > 
> > > > But then it says says VMMs shouldn't set the bit.
> > > > 
> > > >     When emulating the CPUID instruction, a virtual-machine monitor should return
> > > >     this bit as 0 if those pushes can cause VM exits.
> > > > 
> > > > Based on the Xen code (which is sadly a far better source of information than the
> > > > SDM), I *think* that what the SDM is trying to say is that VMMs should not set
> > > > CET_SS if VM-Exits can occur ***and*** the bit is not set in the host CPU.  Because
> > > > if the SDM really means "VMMs should never set the bit", then what on earth is the
> > > > point of the bit.
> > > I need to double check for the vague description.
> > > 
> > >  From my understanding, on bare metal side, if the bit is 1, OS can enable
> > > SSS if pushes won't cause page fault. But for VM case, it's not recommended
> > > (regardless of the bit state) to set the bit as vm-exits caused by guest SSS
> > > pushes cannot be fully excluded.
> > > 
> > > In other word, the bit is mainly for bare metal guidance now.
> > > 
> > > > > In summary, this new series enables CET user SHSTK/IBT and kernel IBT, but
> > > > > doesn't fully support CET supervisor SHSTK, the enabling work is left for
> > > > > the future.
> > > > Why?  If my interpretation of the SDM is correct, then all the pieces are there.
> > ...
> > 
> > > And also based on above SDM description, I don't want to add the support
> > > blindly now.
> > *sigh*
> > 
> > I got filled in on the details offlist.
> > 
> > 1) In the next version of this series, please rework it to reincorporate Supervisor
> >     Shadow Stack support into the main series, i.e. pretend Intel's implemenation
> >     isn't horribly flawed.
> 
> Let me make it clear, you want me to do two things:
> 
> 1)Add Supervisor Shadow Stack  state support(i.e., XSS.bit12(CET_S)) into
> kernel so that host can support guest Supervisor Shadow Stack MSRs in g/h FPU
> context switch.

If that's necessary for correct functionality, yes.

> 2) Add Supervisor Shadow stack support into KVM part so that guest OS is
> able to use SSS with risk.

Yes.  Architecturally, if KVM advertises X86_FEATURE_SHSTK, then KVM needs to
provide both User and Supervisor support.  CET_SSS doesn't change the architecture,
it's little more than a hint.  And even if the guest follows SDM's recommendation
to not enable shadow stacks, a clever kernel can still utilize SSS assets, e.g. use
the MSRs as scratch registers.