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

[Dave Hansen]

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?

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>