[PATCH v2 5/6] powerpc/pkeys: make protection key 0 less special

[Michael Ellerman]

Ram Pai <linuxram at us.ibm.com> writes:
> Applications need the ability to associate an address-range with some
> key and latter revert to its initial default key. Pkey-0 comes close to
> providing this function but falls short, because the current
> implementation disallows applications to explicitly associate pkey-0 to
> the address range.
>
> Lets make pkey-0 less special and treat it almost like any other key.
> Thus it can be explicitly associated with any address range, and can be
> freed. This gives the application more flexibility and power.  The
> ability to free pkey-0 must be used responsibily, since pkey-0 is
> associated with almost all address-range by default.
>
> Even with this change pkey-0 continues to be slightly more special
> from the following point of view.
> (a) it is implicitly allocated.
> (b) it is the default key assigned to any address-range.
> (c) its permissions cannot be modified by userspace.
>
> NOTE: (c) is specific to powerpc only. pkey-0 is associated by default
> with all pages including kernel pages, and pkeys are also active in
> kernel mode. If any permission is denied on pkey-0, the kernel running
> in the context of the application will be unable to operate.

We could fix that by saving/restoring the AMR when we come into the
kernel, and switching to a kernel-AMR with all keys accessible.

We'd then need to think about copy_to/from_user() gup etc. So maybe we
don't want to do that. But it's not set in stone.

Are we calling this a bug fix?

cheers