Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

[Andy Lutomirski <luto@amacapital.net>]

On Dec 22, 2015 2:04 AM, "Dave Hansen" <dave.hansen@linux.intel.com> wrote:
>
> On 12/18/2015 03:16 PM, Andy Lutomirski wrote:
> > Hrm.  We might also want an option to change pkru and/or baseline_pkru
> > in all threads in the current mm.  That's optional but it could be
> > handy.  Maybe it would be as simple as having the allocate-a-pkey call
> > have an option to set an initial baseline value and an option to
> > propagate that initial value to pre-existing threads.
>
> Do you mean actively going in and changing PKRU in other threads?  I
> fear that will be dangerous.
>
> IMNHO, whatever we do, I think we need to ensure that _raw_ PKRU calls
> are allowed (somehow).  Raw in this case would mean a thread calling
> WRPKRU without a system call and without checking in with what any other
> threads are doing.
>
> Let's say baseline_pkru=0x004 (we're access-disabling PKEY[1] and using
> it for execute-only).  Now, a thread is trying to do this:
>
>         pkey2 = sys_pkey_alloc(); // now pkey2=2
>         tmp = rdpkru(); // 0x004
>         tmp |= 0x10; // set PKRU[2].AD=1
>         wrpkru(tmp);
>
> While another thread does:
>
>         pkey4 = pkey_alloc(); // pkey4=4
>         sys_pkey_set(pkey4, ACCESS_DISABLE, SET_BASELINE_ALL_THREADS);
>
> Without some kind of locking, that's going to race.  We could do all the
> locking in the kernel, but that requires that the kernel do all the PKRU
> writing, which I'd really like to avoid.
>
> I think the closest we can get reasonably is to have the kernel track
> the baseline_pkru and then allow userspace to query it in case userspace
> decides that thread needs to update its thread-local PKRU from the baseline.

Yeah, fair point.  Let's skip the modify-other-threads thing.

Perhaps this is silly, but what if the default were changed to deny
reads and writes for unallocated keys?  Is there a use case that
breaks?

--Andy