Re: [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

[Andy Lutomirski]

On Wed, Jun 17, 2020 at 1:33 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Tue, Jun 16, 2020 at 02:17:16PM -0700, Andy Lutomirski wrote:
> > We definitely need to sanitize MXCSR for kernel fpu if kernel fpu
> > means SSE2. If kernel fpu means x87, we need to fix the fpu control
> > word.
>
> Bah, there's no need to beat around the bush - let's just do:
>
>         if (boot_cpu_has(X86_FEATURE_XMM))
>                 ldmxcsr(MXCSR_DEFAULT);
>
>         if (boot_cpu_has(X86_FEATURE_FPU))
>                 asm volatile ("fninit");
>
> and be sure that kernel users get a squeaky-clean FPU.
>
> > On x86_64, I suspect the UEFI ABI technically requires a clean x87
> > control word too. If weâre willing to declare that the kernel proper
> > wonât use x87, then we could shove that into the UEFI code.
>
> Nah, we don't trust the firmware.

What I mean is: if we trust ourselves to have no x87 instructions in
the kernel, we could put the FNINIT in the UEFI stubs to save some
cycles.  I don't know how slow FNINIT is.

>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette



-- 
Andy Lutomirski
AMA Capital Management, LLC