Re: [RFC v5 12/38] mm: ability to disable execute permission on a key at creation

From: Ram Pai
Date: Tue Jul 11 2017 - 17:51:22 EST

On Wed, Jul 12, 2017 at 07:29:37AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2017-07-11 at 11:11 -0700, Dave Hansen wrote:
> > On 07/05/2017 02:21 PM, Ram Pai wrote:
> > > Currently sys_pkey_create() provides the ability to disable read
> > > and write permission on the key, at creation. powerpc has the
> > > hardware support to disable execute on a pkey as well.This patch
> > > enhances the interface to let disable execute at key creation
> > > time. x86 does not allow this. Hence the next patch will add
> > > ability in x86 to return error if PKEY_DISABLE_EXECUTE is
> > > specified.
> That leads to the question... How do you tell userspace.
> (apologies if I missed that in an existing patch in the series)
> How do we inform userspace of the key capabilities ? There are at least
> two things userspace may want to know already:
> - What protection bits are supported for a key

the userspace is the one which allocates the keys and enables/disables the
protection bits on the key. the kernel is just a facilitator. Now if the
use space wants to know the current permissions on a given key, it can
just read the AMR/PKRU register on powerpc/intel respectively.

> - How many keys exist

There is no standard way of finding this other than trying to allocate
as many till you fail. A procfs or sysfs file can be added to expose
this information.

> - Which keys are available for use by userspace. On PowerPC, the
> kernel can reserve some keys for itself, so can the hypervisor. In
> fact, they do.

this information can be exposed through /proc or /sysfs

I am sure there will be more demands and requirements as applications
start leveraging these feature.

> Cheers,
> Ben.

Ram Pai