Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand

[Pawan Gupta]

On Mon, Feb 26, 2024 at 03:52:33PM -0800, Pawan Gupta wrote:
> Macro used for MDS mitigation executes VERW with relative addressing for
> the operand. This is unnecessary and creates a problem for backports on
> older kernels that don't support relocations in alternatives. Relocation
> support was added by commit 270a69c4485d ("x86/alternative: Support
> relocations in alternatives"). Also asm for fixed addressing is much
> more cleaner than relative RIP addressing.
> 
> Simplify the asm by using fixed addressing for VERW operand.
> 
> Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
> Reported-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
> Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@xxxxxxxx/
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/nospec-branch.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 2aa52cab1e46..ab19c7f1167b 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -323,7 +323,7 @@
>   * Note: Only the memory operand variant of VERW clears the CPU buffers.
>   */
>  .macro CLEAR_CPU_BUFFERS
> -	ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
> +	ALTERNATIVE "", __stringify(verw mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF

Extremely sorry this change will not work with KASLR. The patch was
rushed to facilitate backports, and was only tested on qemu that had
KASLR disabled. :( Lesson learnt.

Please drop this patch.