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

Josh Poimboeuf
2023-10-23T18:35:28Z

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