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

[Borislav Petkov]

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.

-- 
Regards/Gruss,
    Boris.

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