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Subject: Re: [PATCH 1/2] x86/pkeys: copy pkey state at fork()
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CA+DvKQLpizz2LJppOts1fB6ujQNqj0fii8mo-UfosoYpQWRb5g@mail.gmail.com>
Date:   Fri, 26 Oct 2018 15:12:17 -0700
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@kernel.org>, jroedel@suse.de
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Message-Id: <FA6429E0-C480-47AA-AAF0-95E615D8265B@amacapital.net>
References: <20181026195146.9C7C1136@viggo.jf.intel.com> <0e5fd8bc-0b18-ea88-ed95-ec81a44d0783@intel.com> <CA+DvKQLpizz2LJppOts1fB6ujQNqj0fii8mo-UfosoYpQWRb5g@mail.gmail.com>
To:     Daniel Micay <danielmicay@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Oct 26, 2018, at 2:39 PM, Daniel Micay <danielmicay@gmail.com> wrote:
>=20
> I ended up working around this with a pthread_atfork handler disabling
> my usage of the feature in the child process for the time being. I
> don't have an easy way to detect if the bug is present within a
> library so

Can you not just make sure that the fix is backported to all relevant kernel=
s?

I suppose we could add a new flag for pkey_get() or something.

> I'm going to need a kernel version check with a table of
> kernel releases fixing the problem for each stable branch.

That won=E2=80=99t work right on district kernels. Please don=E2=80=99t go t=
here.

>=20
> It would be helpful if there was a new cpuinfo flag to check if the
> MPK state is preserved on fork in addition to the existing ospke flag.
> The problem will fade away over time but in my experience there are a
> lot of people using distributions with kernels not incorporating all
> of the stable fixes. I expect other people will run into the problem
> once hardware with MPK is more widely available and other people try
> to use it for various things like moving GC or assorted security
> features. Someone will end up running software adopting it on an older
> kernel with the problem.
>=20
> The clobbering issue I found with MAP_FIXED_NOREPLACE isn't quite
> as annoying because it was easy to make a runtime test usable in a library=

> to see if the feature works properly.