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

On Tue, Jun 19, 2018 at 10:40:08PM +1000, Michael Ellerman wrote:

Ram Pai [off-list ref] writes:

quoted

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?

Actually, I call it borderline bug fix. Its more of a feature.

RP