linux-kernel - Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

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Message-ID: <CALCETrW1qLZE_cq1CvmLkdnFyKRWVZuah29xERTC7o0eZ8DbwQ@mail.gmail.com>
Date:	Mon, 11 Jul 2016 07:45:20 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave@...1.net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Hugh Dickins <hughd@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

On Mon, Jul 11, 2016 at 7:34 AM, Dave Hansen <dave@...1.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.