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

[Josh Poimboeuf]

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