Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition

[Josh Poimboeuf]

On Fri, Oct 20, 2023 at 01:45:03PM -0700, Pawan Gupta wrote:
> @@ -663,6 +665,10 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
>  	/* Restore RDI. */
>  	popq	%rdi
>  	swapgs
> +
> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> +	USER_CLEAR_CPU_BUFFERS
> +
>  	jmp	.Lnative_iret
>  
>  
> @@ -774,6 +780,9 @@ native_irq_return_ldt:
>  	 */
>  	popq	%rax				/* Restore user RAX */
>  
> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> +	USER_CLEAR_CPU_BUFFERS
> +

Can the above two USER_CLEAR_CPU_BUFFERS be replaced with a single one
just above native_irq_return_iret?  Otherwise the native_irq_return_ldt
case ends up getting two VERWs.

>  	/*
>  	 * RSP now points to an ordinary IRET frame, except that the page
>  	 * is read-only and RSP[31:16] are preloaded with the userspace
> @@ -1502,6 +1511,9 @@ nmi_restore:
>  	std
>  	movq	$0, 5*8(%rsp)		/* clear "NMI executing" */
>  
> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> +	USER_CLEAR_CPU_BUFFERS
> +
>  	/*
>  	 * iretq reads the "iret" frame and exits the NMI stack in a
>  	 * single instruction.  We are returning to kernel mode, so this

This isn't needed here.  This is the NMI return-to-kernel path.

The NMI return-to-user path is already mitigated as it goes through
swapgs_restore_regs_and_return_to_usermode.

-- 
Josh