pkeys: Reserve PKEY_DISABLE_READ

[Andy Lutomirski]

> On Dec 2, 2018, at 8:02 PM, Ram Pai <linuxram at us.ibm.com> wrote:
>
>> On Thu, Nov 29, 2018 at 12:37:15PM +0100, Florian Weimer wrote:
>> * Dave Hansen:
>>
>>>> On 11/27/18 3:57 AM, Florian Weimer wrote:
>>>> I would have expected something that translates PKEY_DISABLE_WRITE |
>>>> PKEY_DISABLE_READ into PKEY_DISABLE_ACCESS, and also accepts
>>>> PKEY_DISABLE_ACCESS | PKEY_DISABLE_READ, for consistency with POWER.
>>>>
>>>> (My understanding is that PKEY_DISABLE_ACCESS does not disable all
>>>> access, but produces execute-only memory.)
>>>
>>> Correct, it disables all data access, but not execution.
>>
>> So I would expect something like this (completely untested, I did not
>> even compile this):
>
>
> Ok. I re-read through the entire email thread to understand the problem and
> the proposed solution. Let me summarize it below. Lets see if we are on the same
> plate.
>
> So the problem is as follows:
>
> Currently the kernel supports  'disable-write'  and 'disable-access'.
>
> On x86, cpu supports 'disable-write' and 'disable-access'. This
> matches with what the kernel supports. All good.
>
> However on power, cpu supports 'disable-read' too. Since userspace can
> program the cpu directly, userspace has the ability to set
> 'disable-read' too.  This can lead to inconsistency between the kernel
> and the userspace.
>
> We want the kernel to match userspace on all architectures.
>
> Proposed Solution:
>
> Enhance the kernel to understand 'disable-read', and facilitate architectures
> that understand 'disable-read' to allow it.
>
> Also explicitly define the semantics of disable-access  as
> 'disable-read and disable-write'
>
> Did I get this right?  Assuming I did, the implementation has to do
> the following --
>
>    On power, sys_pkey_alloc() should succeed if the init_val
>    is PKEY_DISABLE_READ, PKEY_DISABLE_WRITE, PKEY_DISABLE_ACCESS
>    or any combination of the three.
>
>    On x86, sys_pkey_alloc() should succeed if the init_val is
>    PKEY_DISABLE_WRITE or PKEY_DISABLE_ACCESS or PKEY_DISABLE_READ
>    or any combination of the three, except  PKEY_DISABLE_READ
>          specified all by itself.
>
>    On all other arches, none of the flags are supported.

I don’t really love having a situation where you can use different
flag combinations to refer to the same mode.

Also, we should document the effect these flags have on execute permission.