Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with WRMSR

[pawan.kumar.gupta@xxxxxxxxxxxxxxx]

On Thu, Oct 06, 2022 at 02:42:10AM +0000, Andrew Cooper wrote:
> On 05/10/2022 23:02, Suraj Jitindar Singh wrote:
> > == Solution ==
> >
> > The WRMSR instruction can be used as a speculation barrier and a serialising
> > instruction. Use this on the VM exit path instead to ensure that a CALL
> > instruction (in this case the call to vmx_spec_ctrl_restore_host) has retired
> > before the prediction of a following unbalanced RET.
>
> While both of these sentences are true statements, you've missed the
> necessary safety property.
>
> One CALL has to retire before *any* RET can execute.
>
> There are several ways the frontend can end up eventually consuming the
> bad RSB entry; they all stem from an execute (not prediction) of the
> next RET instruction.
>
> As to the change, ...
>
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index c9b49a09e6b5..fdcd8e10c2ab 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7049,8 +7049,13 @@ void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
>
> ... out of context above this hunk is:
>
>     if (!cpu_feature_enabled(X86_FEATURE_MSR_SPEC_CTRL))
>         return;
>
> meaning that there is a return instruction which is programmatically
> reachable ahead of the WRMSR.
>
> Whether it is speculatively reachable depends on whether the frontend
> can see through the _static_cpu_has(), as well as
> X86_FEATURE_MSR_SPEC_CTRL never becoming compile time evaluable.

In this case wouldn't _static_cpu_has() be runtime patched to a JMP
(<+8> below) or a NOP? RET (at <+13>) should not be reachable even
speculatively. What am I missing?

Dump of assembler code for function vmx_spec_ctrl_restore_host:

  arch/x86/kvm/vmx/vmx.c:
                u64 hostval = this_cpu_read(x86_spec_ctrl_current);
  		<+0>:     mov    %gs:0x7e022e60(%rip),%r8        # 0x1ad48 <x86_spec_ctrl_current>

  ./arch/x86/include/asm/cpufeature.h:
                asm_volatile_goto(
  		<+8>:     jmp    <vmx_spec_ctrl_restore_host+14>
  		<+10>:    nopl   (%rax)
  		<+13>:    ret

  arch/x86/kvm/vmx/vmx.c:
                if (flags & VMX_RUN_SAVE_SPEC_CTRL)
  		<+14>:    and    $0x2,%esi
  		<+17>:    je     <vmx_spec_ctrl_restore_host+40>
  [...]