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

[Yu-cheng Yu]

On Thu, 2020-07-23 at 11:41 -0700, Dave Hansen wrote:
> On 7/23/20 9:56 AM, Sean Christopherson wrote:
> > On Thu, Jul 23, 2020 at 09:41:37AM -0700, Dave Hansen wrote:
> > > 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? ;)
> > Gah, should have qualified that with "meaningful or measurable negative
> > impacts".  E.g. the extra 40 bytes for CET XSAVE state seems like it would
> > be acceptable overhead, but noticeably increasing the latency of XSAVES
> > and/or XRSTORS would not be acceptable.
> 
> It's 40 bytes, but it's 40 bytes of just pure, unadulterated waste.  It
> would have no *chance* of being used.  It's also quite precisely
> measurable on a given system:
> 
> 	cat /proc/slabinfo | grep task_struct | awk '{print $3 * 40}'

If there is value in getting these two patches merged first, we can move
XFEATURE_MASK_CET_USER to XFEATURE_MASK_SUPERVISOR_UNSUPPORTED for now, until
CET is eventually merged.  That way, there is no space wasted.

Yu-cheng