Re: [PATCH 1/1 v2] x86: pkey-mprotect must allow pkey-0

On 03/14/2018 10:14 AM, Ram Pai wrote:

I look at key-0 as 'the key'. It has special status. 
(a) It always exist.

Do you mean "is always allocated"?

(b) it cannot be freed.

This is the one I'm questioning.

(c) it is assigned by default.

I agree on this totally. :)

(d) its permissions cannot be modified.

Why not?  You could pretty easily get a thread going that had its stack
covered with another pkey and that was being very careful what it
accesses.  It could pretty easily set pkey-0's access or write-disable bits.

(e) it bypasses key-permission checks when assigned.

I don't think this is necessary.  I think the only rule we *need* is:

	pkey-0 is allocated implicitly at execve() time.  You do not
	need to call pkey_alloc() to allocate it.

An arch need not necessarily map 'the key-0' to its key-0.  It could
internally map it to any of its internal key of its choice, transparent
to the application.

I don't understand what you are saying here.