Re: pkeys on POWER: Default AMR, UAMOR values

[Andy Lutomirski <luto@amacapital.net>]

On Fri, May 18, 2018 at 6:17 AM Florian Weimer <fweimer@redhat.com> 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

> * … (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