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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: "Adalbert Lazăr" <alazar@bitdefender.com>
Cc: Yang Weijiang <weijiang.yang@intel.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	pbonzini@redhat.com, jmattson@google.com,
	yu.c.zhang@linux.intel.com, edwin.zhai@intel.com,
	tamas@tklengyel.com, mathieu.tarral@protonmail.com
Subject: Re: [RESEND PATCH v10 06/10] vmx: spp: Set up SPP paging table at vmentry/vmexit
Date: Mon, 13 Jan 2020 13:47:13 -0800
Message-ID: <20200113214713.GG2322@linux.intel.com> (raw)
In-Reply-To: <15789417460.A97E650.22893@host>

On Mon, Jan 13, 2020 at 08:55:46PM +0200, Adalbert Lazăr wrote:
> On Mon, 13 Jan 2020 09:33:58 -0800, Sean Christopherson <sean.j.christopherson@intel.com> 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.
> > 
> > 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().
> > 
> > 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?
> 
> Indeed, we decode the instruction from userspace. I don't know if the
> instruction length helps other projects. Added Tamas and Mathieu.
> 
> In our last VMI API proposal, the breakpoint event had the instruction
> length sent to userspace, but I can't remember why.

INT3 is trap-like, i.e. the VM-Exit occurs after the instruction retires.
It's impossible for software to know how far to unwind RIP without the
instruction length being provided by hardware/KVM, e.g. if the guest is
being silly and prepends ignored prefixes on the INT3.

Self-aware software has a priori knowledge of what's being patched in,
and practically speaking I don't any well-behaved sane software uses
prefixes with INT3, but from a VMM's perspective it's legal and possible.

> 
> https://lore.kernel.org/kvm/20190809160047.8319-62-alazar@bitdefender.com/

  reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-02  6:13 [RESEND PATCH v10 00/10] Enable Sub-Page Write Protection Support Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 01/10] Documentation: Add EPT based Subpage Protection and related APIs Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 02/10] vmx: spp: Add control flags for Sub-Page Protection(SPP) Yang Weijiang
2020-01-10 16:58   ` Sean Christopherson
2020-01-13  5:44     ` Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 03/10] mmu: spp: Add SPP Table setup functions Yang Weijiang
2020-01-10 17:26   ` Sean Christopherson
2020-01-13  6:00     ` Yang Weijiang
2020-01-10 17:40   ` Sean Christopherson
2020-01-13  6:04     ` Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 04/10] mmu: spp: Add functions to operate SPP access bitmap Yang Weijiang
2020-01-10 17:38   ` Sean Christopherson
2020-01-13  6:15     ` Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 05/10] x86: spp: Introduce user-space SPP IOCTLs Yang Weijiang
2020-01-10 18:10   ` Sean Christopherson
2020-01-13  8:21     ` Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 06/10] vmx: spp: Set up SPP paging table at vmentry/vmexit Yang Weijiang
2020-01-10 17:55   ` Sean Christopherson
2020-01-13  6:50     ` Yang Weijiang
2020-01-21 14:01     ` Paolo Bonzini
2020-01-10 18:04   ` Sean Christopherson
2020-01-13  8:10     ` Yang Weijiang
2020-01-13 17:33       ` Sean Christopherson
2020-01-13 18:55         ` Adalbert Lazăr
2020-01-13 21:47           ` Sean Christopherson [this message]
2020-01-14  3:08         ` Yang Weijiang
2020-01-14 18:58           ` Sean Christopherson
2020-01-15  1:36             ` Yang Weijiang
2020-01-21 14:14             ` Paolo Bonzini
2020-01-02  6:13 ` [RESEND PATCH v10 07/10] mmu: spp: Enable Lazy mode SPP protection Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 08/10] mmu: spp: Handle SPP protected pages when VM memory changes Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 09/10] x86: spp: Add SPP protection check in emulation Yang Weijiang
2020-01-02  6:13 ` [RESEND PATCH v10 10/10] kvm: selftests: selftest for Sub-Page protection Yang Weijiang
  -- strict thread matches above, loose matches on Subject: below --
2020-01-02  5:18 [RESEND PATCH v10 00/10] Enable Sub-Page Write Protection Support Yang Weijiang
2020-01-02  5:19 ` [RESEND PATCH v10 06/10] vmx: spp: Set up SPP paging table at vmentry/vmexit Yang Weijiang

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