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

[Sean Christopherson]

On Thu, Jul 23, 2020 at 11:41:55AM -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

Well, technically the guest would be using that space :-).

> measurable on a given system:
> 
> 	cat /proc/slabinfo | grep task_struct | awk '{print $3 * 40}'
> 
> I don't expect it would do *much* to XSAVE/XRSTOR.  There's probably an
> extra conditional and jump in the ucode, but that's probably in the
> noise.  I assume that all the CET state has functioning init and
> modified trackers and we don't do anything to spoil their state.  It
> would be good to check that in practice, though it probably isn't the
> end of the world either way.  We've had some bugs in the past where we
> accidentally took things out of their init state.
> 
> It will make signal entry/return slower since we use a plain XSAVE
> without the init optimization.  But, that's just a single cacheline on
> average and some 0's to write.  Probably not noticeable, including the
> 40 bytes of extra userspace signal stack space.
> 
> I think that puts me in the "mildly annoyed" camp more than "hell no",
> but "mildly annoyed" is pretty much my resting state, so it doesn't
> really move the needle. :)
> 
> Why the urgency, though?
> 
> 	https://windows-internals.com/cet-on-windows/
> 
> ?

No urgency, it'd simply be one less KVM feature for us to be carrying and
refreshing.  And as a sort of general question, I was curious if folks
would be open to merging KVM support before kernel.