linux-kernel - Re: [RESEND PATCH v10 06/10] vmx: spp: Set up SPP paging table at vmentry/vmexit

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Date:   Tue, 14 Jan 2020 10:58:08 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, jmattson@...gle.com,
        yu.c.zhang@...ux.intel.com, alazar@...defender.com,
        edwin.zhai@...el.com
Subject: Re: [RESEND PATCH v10 06/10] vmx: spp: Set up SPP paging table at
 vmentry/vmexit

On Tue, Jan 14, 2020 at 11:08:20AM +0800, Yang Weijiang wrote:
> On Mon, Jan 13, 2020 at 09:33:58AM -0800, Sean Christopherson wrote:
> > On Mon, Jan 13, 2020 at 04:10:50PM +0800, Yang Weijiang wrote:
> > > On Fri, Jan 10, 2020 at 10:04:59AM -0800, Sean Christopherson wrote:
> > > > On Thu, Jan 02, 2020 at 02:13:15PM +0800, Yang Weijiang wrote:
> > > > > @@ -3585,7 +3602,30 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
> > > > >  		if ((error_code & PFERR_WRITE_MASK) &&
> > > > >  		    spte_can_locklessly_be_made_writable(spte))
> > > > >  		{
> > > > > -			new_spte |= PT_WRITABLE_MASK;
> > > > > +			/*
> > > > > +			 * Record write protect fault caused by
> > > > > +			 * Sub-page Protection, let VMI decide
> > > > > +			 * the next step.
> > > > > +			 */
> > > > > +			if (spte & PT_SPP_MASK) {
> > > > > +				int len = kvm_x86_ops->get_inst_len(vcpu);
> > > > 
> > > > There's got to be a better way to handle SPP exits than adding a helper
> > > > to retrieve the instruction length.
> > > >
> > > The fault instruction was skipped by kvm_skip_emulated_instruction()
> > > before, but Paolo suggested leave the re-do or skip option to user-space
> > > to make it flexible for write protection or write tracking, so return
> > > length to user-space.
> > 
> > Sorry, my comment was unclear.  I have no objection to punting the fault
> > to userspace, it's the mechanics of how it's done that I dislike.
> > 
> > Specifically, (a) using run->exit_reason to propagate the SPP exit up the
> > stack, e.g. instead of modifying affected call stacks to play nice with
> > any exit to userspace, (b) assuming ->get_insn_len() will always be
> > accurate, e.g. see the various caveats in skip_emulated_instruction() for
> > both VMX and SVM, and (c) duplicating the state capture code in every
> > location that can encounter a SPP fault.
>
> How about calling skip_emulated_instruction() in KVM before exit to

I'm confused.  It sounds like KVM_EXIT_SPP provides the instruction length
because it skips an instruction before exiting to userspace.  But if KVM
is is emulating an instruction, it shouldn't be doing
{kvm_}skip_emulated_instruction(), e.g. if emulation fails due to a SPP
violation (returns KVM_EXIT_SPP) then GUEST_RIP should still point at the
exiting instruction.  Ditto for the fast_page_fault() case, RIP shouldn't
be advanced.

What am I missing?

> userspace, but still return the skipped instruction length, if userspace
> would like to re-execute the instruction, it can unwind RIP or simply
> rely on KVM?

I'm not convinced the instruction length needs to be provided to userspace
for this case.  Obviously it's not difficult to provide the info, I just
don't understand the value added by doing so.  As above, RIP shouldn't
need to be unwound, and blindly skipping an instruction seems like an odd
thing for a VMI engine to do.

> > What I'm hoping is that it's possible to modify the call stacks to
> > explicitly propagate an exit to userspace and/or SPP fault, and shove all
> > the state capture into a common location, e.g. handle_ept_violation().
> >
> The problem is, the state capture code in fast_page_fault() and
> emulation case share different causes, the former is generic occurence
> of SPP induced EPT violation, the latter is atually a "faked" one while
> detecting emulation instruction is writing some SPP protected area, so I
> seperated them.

Can we make SPP dependent on unrestricted guest so that the only entry
point to the emulator is through handle_ept_violation()?  And thus the
only path to triggering KVM_EXIT_SPP would also be through
handle_ept_violation(); (I think, might be forgetting a different emulation
path).

>
> > Side topic, assuming the userspace VMI is going to be instrospecting the
> > faulting instruction, won't it decode the instruction?  I.e. calculate
> > the instruction length anyways?