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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()
Date:   Tue, 16 Jun 2020 14:17:16 -0700
Message-Id: <1D569B6B-B8C3-497E-8A74-2E1A3D46299E@amacapital.net>
References: <20200616180123.GL13515@zn.tnic>
Cc:     x86-ml <x86@kernel.org>, jpa@kernelbug.mail.kapsi.fi,
        Dave Hansen <dave.hansen@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        lkml <linux-kernel@vger.kernel.org>
In-Reply-To: <20200616180123.GL13515@zn.tnic>
To:     Borislav Petkov <bp@alien8.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jun 16, 2020, at 11:01 AM, Borislav Petkov <bp@alien8.de> wrote:
>=20
> =EF=BB=BFOn Tue, Jun 16, 2020 at 09:53:39AM -0700, Andy Lutomirski wrote:
>>> On Tue, Jun 16, 2020 at 2:53 AM Borislav Petkov <bp@alien8.de> wrote:
>>>=20
>>> Ok,
>>>=20
>>> here's the fix first so that it goes in. I'll hammer on the test case la=
ter.
>>=20
>> Does the 32-bit case need FNINIT?
>=20
> Pasting from IRC:
>=20
> I'm thinking if you'd need to reinit the FPU, then you need to do it for
> both, not only 32-bit or do you mean something else? Also, if you end up
> doing FNSAVE (old CPU) that one reinits state.

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.

On x86_64, I suspect the UEFI ABI technically requires a clean x87 control w=
ord too. If we=E2=80=99re willing to declare that the kernel proper won=E2=80=
=99t use x87, then we could shove that into the UEFI code.

>=20
> Whatever we decide doing, this should be a separate patch anyway.
>=20
> Thx.
>=20
> --=20
> Regards/Gruss,
>    Boris.
>=20
> https://people.kernel.org/tglx/notes-about-netiquette