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

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?