[PATCH v3] powerpc, pkey: make protection key 0 less special

[Ram Pai]

On Sat, May 05, 2018 at 02:39:56PM +0200, Michal Suchánek wrote:
> On Fri, 4 May 2018 14:45:07 -0700
> Ram Pai <linuxram at us.ibm.com> wrote:
> 
> > On Fri, May 04, 2018 at 02:31:10PM -0700, Dave Hansen wrote:
> > > On 05/04/2018 02:26 PM, Michal Suchánek wrote:  
> > > > If it is not ok to change permissions of pkey 0 is it ok to free
> > > > it?  
> > > 
> > > It's pretty much never OK to free it on x86 or ppc.  But, we're not
> > > going to put code in to keep userspace from shooting itself in the
> > > foot, at least on x86.  
> > 
> > and on powerpc aswell.
> 
> But once it's free it can be re-allocated. So you are moving the
> special-casing from free code to code dealing with allocation.

Actually if an application frees key-0, it has potentially opened up a
can-of-worms. It could step on anything that explodes. 

Its choice between imposing policies on an application v/s freeing it up
to choose its own policy. I think the kernel should impose some form of
mild-policy. But others think there should be none.

> 
> If you want something like allocate_exec_only_pkey then the function
> (either in kernel or in userspace) needs to make sure it is not
> getting/requesting key 0 on powerpc.

Yes. makes sense. I will put in some checks towards that.


Thanks,
RP

> 
> Thanks
> 
> Michal

-- 
Ram Pai