linux-kernel - Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW operand

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Message-ID: <20241009073437.GG17263@noisy.programming.kicks-ass.net>
Date: Wed, 9 Oct 2024 09:34:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
	Robert Gill <rtgill82@...il.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>, stable@...r.kernel.org,
	#@...-bot2.tec.linutronix.de, 5.10+@...-bot2.tec.linutronix.de,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Brian Gerst <brgerst@...il.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org
Subject: Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW
 operand

On Wed, Oct 09, 2024 at 08:11:02AM +0200, Borislav Petkov wrote:
> On Tue, Oct 08, 2024 at 10:45:36PM -0000, tip-bot2 for Pawan Gupta wrote:
> >  .macro CLEAR_CPU_BUFFERS
> > -	ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
> > +#ifdef CONFIG_X86_64
> > +	ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
> > +#else
> > +	/*
> > +	 * In 32bit mode, the memory operand must be a %cs reference. The data
> > +	 * segments may not be usable (vm86 mode), and the stack segment may not
> > +	 * be flat (ESPFIX32).
> > +	 */
> > +	ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
> > +#endif
> 
> So why didn't we ifdef the "verw mds_verw_sel(%rip)" and "verw
> %cs:mds_verw_sel" macro argument instead of adding more bigger ugly ifdeffery?

You need ifdeffery either way around, either directly like this or for
that macro. This is simple and straight forward.