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Message-ID: <45417907-94c4-4243-9e68-d68d0b34ed5c@intel.com>
Date:   Thu, 26 Oct 2023 15:17:48 -0700
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Subject: Re: [PATCH v3 2/6] x86/entry_64: Add VERW just before userspace
 transition
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To:     Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc:     Nikolay Borisov <nik.borisov@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, tony.luck@intel.com,
        ak@linux.intel.com, tim.c.chen@linux.intel.com,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        kvm@vger.kernel.org,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Daniel Sneddon <daniel.sneddon@linux.intel.com>,
        antonio.gomez.iglesias@linux.intel.com
References: <20231025-delay-verw-v3-0-52663677ee35@linux.intel.com>
 <20231025-delay-verw-v3-2-52663677ee35@linux.intel.com>
 <2cda7e85-aa75-4257-864d-0092b3339e0e@suse.com>
 <20231026192950.ylzc66f3f5naqvjv@desk>
 <ae3d993f-6ce4-42de-b9c4-ef0c7db663c0@intel.com>
 <20231026211508.tmd7hfniesiu53ps@desk> <20231026221311.5dqmnmvq4pnpqswn@desk>
From:   Dave Hansen <dave.hansen@intel.com>
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In-Reply-To: <20231026221311.5dqmnmvq4pnpqswn@desk>
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Precedence: bulk

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.