Re: [PATCH 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation

[Pawan Gupta]

On Mon, Oct 23, 2023 at 07:58:57AM -0700, Sean Christopherson wrote:
> On Fri, Oct 20, 2023, Pawan Gupta wrote:
> > On Fri, Oct 20, 2023 at 03:55:07PM -0700, Sean Christopherson wrote:
> > > On Fri, Oct 20, 2023, Pawan Gupta wrote:
> > > > During VMentry VERW is executed to mitigate MDS. After VERW, any memory
> > > > access like register push onto stack may put host data in MDS affected
> > > > CPU buffers. A guest can then use MDS to sample host data.
> > > > 
> > > > Although likelihood of secrets surviving in registers at current VERW
> > > > callsite is less, but it can't be ruled out. Harden the MDS mitigation
> > > > by moving the VERW mitigation late in VMentry path.
> > > > 
> > > > Note that VERW for MMIO Stale Data mitigation is unchanged because of
> > > > the complexity of per-guest conditional VERW which is not easy to handle
> > > > that late in asm with no GPRs available. If the CPU is also affected by
> > > > MDS, VERW is unconditionally executed late in asm regardless of guest
> > > > having MMIO access.
> > > > 
> > > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> > > > ---
> > > >  arch/x86/kvm/vmx/vmenter.S |  9 +++++++++
> > > >  arch/x86/kvm/vmx/vmx.c     | 10 +++++++---
> > > >  2 files changed, 16 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> > > > index be275a0410a8..efa716cf4727 100644
> > > > --- a/arch/x86/kvm/vmx/vmenter.S
> > > > +++ b/arch/x86/kvm/vmx/vmenter.S
> > > > @@ -1,6 +1,7 @@
> > > >  /* SPDX-License-Identifier: GPL-2.0 */
> > > >  #include <linux/linkage.h>
> > > >  #include <asm/asm.h>
> > > > +#include <asm/segment.h>
> > > >  #include <asm/bitsperlong.h>
> > > >  #include <asm/kvm_vcpu_regs.h>
> > > >  #include <asm/nospec-branch.h>
> > > > @@ -31,6 +32,8 @@
> > > >  #define VCPU_R15	__VCPU_REGS_R15 * WORD_SIZE
> > > >  #endif
> > > >  
> > > > +#define GUEST_CLEAR_CPU_BUFFERS		USER_CLEAR_CPU_BUFFERS
> > > > +
> > > >  .macro VMX_DO_EVENT_IRQOFF call_insn call_target
> > > >  	/*
> > > >  	 * Unconditionally create a stack frame, getting the correct RSP on the
> > > > @@ -177,10 +180,16 @@ SYM_FUNC_START(__vmx_vcpu_run)
> > > >   * the 'vmx_vmexit' label below.
> > > >   */
> > > >  .Lvmresume:
> > > > +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> > > > +	GUEST_CLEAR_CPU_BUFFERS
> > > 
> > > I have a very hard time believing that it's worth duplicating the mitigation
> > > for VMRESUME vs. VMLAUNCH just to land it after a Jcc.
> > 
> > VERW modifies the flags, so it either needs to be after Jcc or we
> > push/pop flags that adds 2 extra memory operations. Please let me know
> > if there is a better option.
> 
> Ugh, I assumed that piggybacking VERW overrode the original behavior entirely, I
> didn't realize it sacrifices EFLAGS.ZF on the altar of mitigations.
> 
> Luckily, this is easy to solve now that VMRESUME vs. VMLAUNCH uses a flag instead
> of a dedicated bool.

Thats great.

> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Date: Mon, 23 Oct 2023 07:44:35 -0700
> Subject: [PATCH] KVM: VMX: Use BT+JNC, i.e. EFLAGS.CF to select VMRESUME vs.
>  VMLAUNCH
> 
> Use EFLAGS.CF instead of EFLAGS.ZF to track whether to use VMRESUME versus
> VMLAUNCH.  Freeing up EFLAGS.ZF will allow doing VERW, which clobbers ZF,
> for MDS mitigations as late as possible without needing to duplicate VERW
> for both paths.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Thanks for the patch, I will include it in the next revision.