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

[Peter Zijlstra]

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.