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

Josh Poimboeuf
2023-10-23T21:47:58Z

On Mon, Oct 23, 2023 at 02:04:10PM -0700, Pawan Gupta wrote:
> On Mon, Oct 23, 2023 at 11:35:21AM -0700, Josh Poimboeuf wrote:
> > 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.
>
> Wouldn't that make interrupts returning to kernel also execute VERWs?
>
> idtentry_body
> error_return
> restore_regs_and_return_to_kernel
> verw
>
> native_irq_return_ldt doesn't look to be a common case. Anyways, I will
> see how to remove the extra VERW.

Ah, right.

> > > /*
> > > * 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.
>
> Yes, the VERW here can be omitted. But probably need to check if an NMI
> occuring between VERW and ring transition will still execute VERW after
> the NMI.

That window does exist, though I'm not sure it's worth worrying about.

--
Josh