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

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Date:   Sat, 17 Mar 2018 09:01:20 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linuxram@...ibm.com, mpe@...erman.id.au, mingo@...nel.org,
        akpm@...ux-foundation.org, shuah@...nel.org
Subject: Re: [PATCH 1/3] x86, pkeys: do not special case protection key 0

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.