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 15:17:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.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 10/26/23 15:13, Pawan Gupta wrote:
>>>> 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.

Right.

This is also a path where we care *ZERO* about performance.  It's
basically all upside to _add_ VERW and all downside (increased attack
surface) to skip it.