Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host()

From: Josh Poimboeuf
Date: Wed May 31 2023 - 20:54:02 EST

On Thu, Jun 01, 2023 at 01:29:12AM +0100, Andrew Cooper wrote:
> On 31/05/2023 4:01 pm, Jon Kohler wrote:
> > Remove barrier_nospec(), which translates to LFENCE, in
> > vmx_spec_ctrl_restore_host() as RSB-barriers (as defined by [1])
> > already exist prior to this point.
> >
> > This LFENCE was added on commit fc02735b14ff ("KVM: VMX: Prevent guest
> > RSB poisoning attacks with eIBRS") in the 5.19 timeframe; however,
> > commit 2b1299322016 ("x86/speculation: Add RSB VM Exit protections") in
> > 6.0 timeframe added a LFENCE for X86_FEATURE_RSB_VMEXIT_LITE was added
> > directly in vmx_vmexit, prior to CALL vmx_spec_ctrl_restore_host.
> >
> > For posterity, vmx_spec_ctrl_restore_host also will execute WRMSR to
> > IA32_SPEC_CTRL for X86_FEATURE_KERNEL_IBRS or when guest/host MSR value
> > does not match, which serves as an additional RSB-barrier.
> >
> > [1]
> Yeah, unfortunately PBRSB is insidious.
> From memory (please correct me if I'm wrong), the required safety
> property is this:  After a VMExit (if IBRS was set prior to exit) or the
> write to MSR_SPEC_CTRL setting IBRS (if IBRS was not set prior to exit),
> one single CALL instruction must architecturally retire before any RET
> instructions execute (speculatively).
> There are several ways to arrange this, but they all basically boil down
> to having some serialising instruction between the first CALL and first
> RET on any reachable path from VMExit.

The document says the problem is *unbalanced* RET, i.e. RSB underflow.

So the mitigation needs a single RSB stuff (i.e., unbalanced CALL) and
then an LFENCE anytime before the next unbalanced RET.