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Subject: Re: [PATCH 1/3] x86, pkeys: do not special case protection key 0
References: <20180316214654.895E24EC@viggo.jf.intel.com>
 <20180316214656.0E059008@viggo.jf.intel.com>
 <alpine.DEB.2.21.1803171011100.1509@nanos.tec.linutronix.de>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <6e0c687d-f465-5433-10be-db04489278a9@intel.com>
Date: Sat, 17 Mar 2018 09:01:20 -0700
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To: Thomas Gleixner <tglx@linutronix.de>, Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxram@us.ibm.com, mpe@ellerman.id.au, mingo@kernel.org, akpm@linux-foundation.org, shuah@kernel.org

On 03/17/2018 02:12 AM, Thomas Gleixner wrote:
>> This is a bit nicer than what Ram proposed because it is simpler
>> and removes special-casing for pkey 0.  On the other hand, it does
>> allow applciations to pkey_free() pkey-0, but that's just a silly
>> thing to do, so we are not going to protect against it.
> What's the consequence of that? Application crashing and burning itself or
> something more subtle?

You would have to:

	pkey_free(0)
	... later
	new_key = pkey_alloc();
	// now new_key=0
	pkey_deny_access(new_key); // or whatever

At which point most apps would probably croak because its stack is
inaccessible.  The free itself does not make the key inaccessible, *but*
we could also do that within the existing ABI if we want.  I think I
called out that behavior as undefined in the manpage.