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

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Date:   Mon, 23 Oct 2023 12:09:49 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Andrew Cooper <andrew.cooper3@...rix.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>,
        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 Mon, Oct 23, 2023 at 11:08:06AM -0700, Josh Poimboeuf wrote:
> On Fri, Oct 20, 2023 at 07:21:34PM -0700, Pawan Gupta wrote:
> > On Sat, Oct 21, 2023 at 02:33:47AM +0100, Andrew Cooper wrote:
> > > 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.
> > 
> > Sorry, I raised the wrong problem. In 64-bit mode, verww only has 32-bit
> > of relative addressing, so memory operand has to be within 4GB of
> > callsite. That could be a constraint.
> 
> Even on x86-64, modules are mapped within 4GB of the kernel, so I don't
> think that's a concern.

You are correct, modules are indeed mapped within 4GB of the kernel. So
what Andrew suggested is feasible. Is that your preference?