Re: [PATCH 1/3] x86, pkeys: do not special case protection key 0

[Dave Hansen <dave.hansen@intel.com>]

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.