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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 11 Jul 2016 07:45:20 -0700
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Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls
To: Dave Hansen <dave@sr71.net>
Cc: Ingo Molnar <mingo@kernel.org>, linux-arch <linux-arch@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux API <linux-api@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Hugh Dickins <hughd@google.com>, "H. Peter Anvin" <hpa@zytor.com>,
        X86 ML <x86@kernel.org>, Dave Hansen <dave.hansen@linux.intel.com>
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On Mon, Jul 11, 2016 at 7:34 AM, Dave Hansen <dave@sr71.net> wrote:
> On 07/10/2016 09:25 PM, Andy Lutomirski wrote:
>> 2. When thread A allocates a pkey, how does it lock down thread B?
>>
>> #2 could be addressed by using fully-locked-down as the initial state
>> post-exec() and copying the state on clone().  Dave, are there any
>> cases in practice where one thread would allocate a pkey and want
>> other threads to immediately have access to the memory with that key?
>
> The only one I can think of is a model where pkeys are used more in a
> "denial" mode rather than an "allow" mode.
>
> For instance, perhaps you don't want to modify your app to use pkeys,
> except for a small routine where you handle untrusted user data.  You
> would, in that routine, deny access to a bunch of keys, but otherwise
> allow access to all so you didn't have to change any other parts of the app.
>
> Should we instead just recommend to userspace that they lock down access
> to keys by default in all threads as a best practice?

Is that really better than doing it in-kernel?  My concern is that
we'll find library code that creates a thread, and that code could run
before the pkey-aware part of the program even starts running.  So how
is user code supposed lock down all of its threads?

seccomp has TSYNC for this, but I don't think that PKRU allows
something like that.