Re: pkeys on POWER: Default AMR, UAMOR values

On Fri, May 18, 2018 at 6:17 AM Florian Weimer [off-list ref] wrote:

I'm working on adding POWER pkeys support to glibc.  The coding work is
done, but I'm faced with some test suite failures.

Unlike the default x86 configuration, on POWER, existing threads have
full access to newly allocated keys.

Or, more precisely, in this scenario:

* Thread A launches thread B
* Thread B waits
* Thread A allocations a protection key with pkey_alloc
* Thread A applies the key to a page
* Thread A signals thread B
* Thread B starts to run and accesses the page

Then at the end, the access will be granted.

I hope it's not too late to change this to denied access.

Furthermore, I think the UAMOR value is wrong as well because it
prevents thread B at the end to set the AMR register.  In particular, if
I do this

* =E2=80=A6 (as before)
* Thread A signals thread B
* Thread B sets the access rights for the key to PKEY_DISABLE_ACCESS
* Thread B reads the current access rights for the key

then it still gets 0 (all access permitted) because the original UAMOR
value inherited from thread A prior to the key allocation masks out the
access right update for the newly allocated key.

This type of issue is why I think that a good protection key ISA would not
have a usermode read-the-whole-register or write-the-whole-register
operation at all.  It's still not clear to me that there is any good
kernel-mode solution.  But at least x86 defaults to deny-everything, which
is more annoying but considerably safer than POWER's behavior.

--Andy