linux-kernel - Re: [PATCH 1/2] x86/fpu: Reset MXCSR to default in kernel_fpu

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Message-ID: <CALCETrXZhFJGJA2h4zP743KYTtni-rQSUME8mtSYUdk1-ZTauQ@mail.gmail.com>
Date:   Fri, 19 Jun 2020 11:01:44 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>, jpa@...nelbug.mail.kapsi.fi,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

On Fri, Jun 19, 2020 at 10:41 AM Borislav Petkov <bp@...en8.de> wrote:
>
> From: Petteri Aimonen <jpa@....mail.kapsi.fi>
>
> Previously, kernel floating point code would run with the MXCSR control
> register value last set by userland code by the thread that was active
> on the CPU core just before kernel call. This could affect calculation
> results if rounding mode was changed, or a crash if a FPU/SIMD exception
> was unmasked.
>
> Restore MXCSR to the kernel's default value.
>
>  [ bp: Carve out from a bigger patch by Petteri, add feature check, add
>    FNINIT call too (amluto). ]

Acked-by: Andy Lutomirski <luto@...nel.org>

but:

shouldn't kernel_fpu_begin() end with a barrier()?