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Subject: Re: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CA+55aFy3tiDKKaE=GR0y9nxpwxJjD_NEU4yNN5YfaOUTFQvG=g@mail.gmail.com>
Date:   Wed, 29 Nov 2017 13:25:09 -0800
Cc:     Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Jarkko Nikula <jarkko.nikula@linux.intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Borislav Petkov <bp@alien8.de>
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Message-Id: <31643632-8A3F-46AC-95AB-27FC94ED79A3@amacapital.net>
References: <0fede9f9-88b0-a6e7-1027-dfb2019b8ef2@linux.intel.com> <20171129070951.hjjjpbyilzaak4ig@gmail.com> <CALCETrXwM7syieKz8CcFE8qHL6r_yA0jP49A1EJsw5UNWz+UNA@mail.gmail.com> <CALCETrXaCtrzu7xyR+wHvSQhMoP4onUg0hfHHoxd72QSCqgkmw@mail.gmail.com> <CA+55aFy3tiDKKaE=GR0y9nxpwxJjD_NEU4yNN5YfaOUTFQvG=g@mail.gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 29, 2017, at 12:58 PM, Linus Torvalds <torvalds@linux-foundation.or=
g> wrote:
>=20
>> On Wed, Nov 29, 2017 at 10:12 AM, Andy Lutomirski <luto@kernel.org> wrote=
:
>>=20
>> Jarkko, can you try the attached patch?  If it survives resume, can
>> you see if the log contains anything interesting?
>=20
> I'm not Jarkko, but I'm not a huge fan of that patch.
>=20
> If this was the cause of the problem (and it looks likely), wouldn't
> it be nicer to instead make sure that __restore_processor_state() is
> made to use only low-level code and easy to verify?
>=20
> That function is already marked "notrace" because it is so fragile,
> and it does the segment register reloads manually with inline asms.

I completely agree, and I think it might be better to move more of that crap=
 to asm.  Also, it looks quite buggy -- it restores segment registers before=
 it loads the LDT, so they had better not be user registers.

Or we could load fixed values into the segment regs if they're not user valu=
es.

>=20
> Could we make it use "native_load_gs_index()" instead? Or even go all
> the way and make it do that user-space %gs load internally with inline
> asm, the way it already does the kernel space %gs?

Dunno.  If we need the exception handling, it can't be inlined.

Anyway, this wasn't meant to be an upstreamable fix.  It was meant to make s=
ure the problem I'm fixing is the right problem.

>=20
> (Maybe "native_wrmsrl()" too?)
>=20
> Or is this actually all supposed to work even under PV? That sounds really=
 iffy.
>=20
>               Linus

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