Re: [PATCH v4] x86/entry_32: Use stack segment selector for VERW operand

[Pawan Gupta]

On Thu, Jul 11, 2024 at 11:03:29AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 10, 2024 at 12:06:47PM -0700, Pawan Gupta wrote:
> > +/*
> > + * Safer version of CLEAR_CPU_BUFFERS that uses %ss to reference VERW operand
> > + * mds_verw_sel. This ensures VERW will not #GP for an arbitrary user %ds.
> > + */
> > +.macro CLEAR_CPU_BUFFERS_SAFE
> > +	ALTERNATIVE "jmp .Lskip_verw\@", "", X86_FEATURE_CLEAR_CPU_BUF
> > +	verw	%ss:_ASM_RIP(mds_verw_sel)
> > +.Lskip_verw\@:
> > +.endm
> 
> I know this is somewhat of a common pattern, but I think it is silly in
> this case. Since we already have the ALTERNATIVE() why not NOP the one
> VERW instruction instead?
> 
> That is,
> 
> 	ALTERNATIVE("", "verw %ss:_ASM_RIP(mds_verw_sel)", X86_FEATURE_CLEAR_CPU_BUF)

Will do, thanks.