linux-kernel - Re: [PATCH v3 2/6] x86/entry_64: Add VERW just before userspace transition

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Date:   Thu, 26 Oct 2023 15:13:11 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Nikolay Borisov <nik.borisov@...e.com>,
        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>,
        Josh Poimboeuf <jpoimboe@...nel.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
Subject: Re: [PATCH v3 2/6] x86/entry_64: Add VERW just before userspace
 transition

On Thu, Oct 26, 2023 at 02:15:11PM -0700, Pawan Gupta wrote:
> On Thu, Oct 26, 2023 at 12:40:49PM -0700, Dave Hansen wrote:
> > On 10/26/23 12:29, Pawan Gupta wrote:
> > > On Thu, Oct 26, 2023 at 07:25:27PM +0300, Nikolay Borisov wrote:
> > >> On 25.10.23 г. 23:52 ч., Pawan Gupta wrote:
> > >>> @@ -1520,6 +1530,7 @@ SYM_CODE_START(ignore_sysret)
> > >>>   	UNWIND_HINT_END_OF_STACK
> > >>>   	ENDBR
> > >>>   	mov	$-ENOSYS, %eax
> > >>> +	CLEAR_CPU_BUFFERS
> > >> nit: Just out of curiosity is it really needed in this case or it's doesn
> > >> for the sake of uniformity so that all ring3 transitions are indeed
> > >> covered??
> > > Interrupts returning to kernel don't clear the CPU buffers. I believe
> > > interrupts will be enabled here, and getting an interrupt here could
> > > leak the data that interrupt touched.
> > 
> > Specifically NMIs, right?
> 
> Yes, and VERW can omitted for the same reason as NMI returning to
> kernel.

Thinking more on this, we should not omit verw here, as this spot is way
easier to target NMIs. A user executing SYSENTER in a loop has much
higher chances of causing an NMI to return to kernel, and skip verw.