Re: [PATCH] x86/fpu/xstate: Fix PKRU covert channel

[Jim Mattson]

On Thu, Aug 31, 2023 at 12:12 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 8/30/23 21:32, Jim Mattson wrote:
> > When XCR0[9] is set, PKRU can be read and written from userspace with
> > XSAVE and XRSTOR, even when CR4.PKE is clear.
> >
> > Clear XCR0[9] when protection keys are disabled.
> >
> > Reported-by: Tavis Ormandy <taviso@xxxxxxxxxx>
> > Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
>
> Is there any way to trigger this other than "nopku" on the command-line?

Or by configuration option:

CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS=n

> I'm not sure how scary this particular covert channel is, but it does
> make sense to do this even if it's only to avoid wasting XSAVE space on
> a feature that nobody can use (for things other than covert channels).
>
> Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>