linux-kernel - Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

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Message-ID: <bdfefc38-c010-4423-b129-3f153078fd67@citrix.com>
Date:   Sat, 21 Oct 2023 02:33:47 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
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>,
        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,
        Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

On 21/10/2023 2:18 am, Pawan Gupta wrote:
> On Sat, Oct 21, 2023 at 12:55:45AM +0100, Andrew Cooper wrote:
>> Also it avoids playing games with hiding data inside an instruction.
>> It's a neat trick, but the neater trick is avoid it whenever possible.
> Thanks for the pointers. I think verw in 32-bit mode won't be able to
> address the operand outside of 4GB range.

And?  In a 32bit kernel, what lives outside of a 4G range?

> Maybe this is fine or could it
> be a problem addressing from e.g. KVM module?

RIP-relative addressing is disp32.  Which is the same as it is for
direct calls.

So if your module is far enough away for VERW to have issues, you've got
far more basic problems to solve first.

~Andrew