linux-kernel - Re: [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu

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Date:   Wed, 17 Jun 2020 10:33:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     x86-ml <x86@...nel.org>, jpa@...nelbug.mail.kapsi.fi,
        Dave Hansen <dave.hansen@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

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 we’re willing to declare that the kernel proper
> won’t use x87, then we could shove that into the UEFI code.

Nah, we don't trust the firmware.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette