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

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Date:   Mon, 23 Oct 2023 15:30:59 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kvm@...r.kernel.org,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH  2/6] x86/entry_64: Add VERW just before userspace
 transition

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.