Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition

[Pawan Gupta]

On Mon, Oct 23, 2023 at 02:47:52PM -0700, Josh Poimboeuf wrote:
> > > >  	/*
> > > >  	 * RSP now points to an ordinary IRET frame, except that the page
> > > >  	 * is read-only and RSP[31:16] are preloaded with the userspace
> > > > @@ -1502,6 +1511,9 @@ nmi_restore:
> > > >  	std
> > > >  	movq	$0, 5*8(%rsp)		/* clear "NMI executing" */
> > > >  
> > > > +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> > > > +	USER_CLEAR_CPU_BUFFERS
> > > > +
> > > >  	/*
> > > >  	 * iretq reads the "iret" frame and exits the NMI stack in a
> > > >  	 * single instruction.  We are returning to kernel mode, so this
> > > 
> > > This isn't needed here.  This is the NMI return-to-kernel path.
> > 
> > Yes, the VERW here can be omitted. But probably need to check if an NMI
> > occuring between VERW and ring transition will still execute VERW after
> > the NMI.
> 
> That window does exist, though I'm not sure it's worth worrying about.

I am in favor of omitting the VERW here, unless someone objects with a
rationale. IMO, precisely timing the NMIs in such a narrow window is
impractical.