Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

[Ram Pai <linuxram@us.ibm.com>]

On Fri, Mar 09, 2018 at 07:37:04PM +1100, Balbir Singh wrote:
> On Fri, Mar 9, 2018 at 7:12 PM, Ram Pai <linuxram@us.ibm.com> wrote:
> > Once an address range is associated with an allocated pkey, it cannot be
> > reverted back to key-0. There is no valid reason for the above behavior.  On
> > the contrary applications need the ability to do so.
> >
> > The patch relaxes the restriction.
> 
> I looked at the code and my observation was going to be that we need
> to change mm_pkey_is_allocated. I still fail to understand what
> happens if pkey 0 is reserved? What is the default key is it the first
> available key? Assuming 0 is the default key may work and seems to
> work, but I am sure its mostly by accident. It would be nice, if we
> could have  a notion of the default key. I don't like the special
> meaning given to key 0 here. Remember on powerpc if 0 is reserved and
> UAMOR/AMOR does not allow modification because it's reserved, setting
> 0 will still fail

The linux pkey API, assumes pkey-0 is the default key. If no key is
explicitly associated with a page, the default key gets associated.
When a default key gets associated with a page, the permissions on the
page are not dictated by the permissions of the default key, but by the
permission of other bits in the pte; i.e _PAGE_RWX.

On powerpc, and AFAICT on x86, neither the hardware nor the hypervisor
reserves key-0. Hence the OS is free to use the key value, the
way it chooses. On Linux we choose to associate key-0 the special status
called default-key.

However I see your point. If some cpu architecture takes away key-0 from
Linux, than implementing the special status for key-0 on that
architecture can become challenging, though not impossible. That
architecture implementation can internally map key-0 value to some other
available key, and associate that key to the page. And offcourse make
sure that the hardware/MMU uses the pte's RWX bits to enforce
permissions, for that key.


-- 
Ram Pai