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

[Peter Zijlstra]

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)

and call it a day?