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

[Andy Lutomirski]

On Mon, Dec 21, 2015 at 3:04 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 12/18/2015 02:28 PM, Andy Lutomirski wrote:
> ...
>>> I could imagine that some kernel person would want to use even more
>>> keys, but I think two fixed keys are kind of the minimal we'd want to
>>> use.
>>
>> I imagine we'd reserve key 0 for normal page and key 1 for deny-read.
>> Let me be a bit more concrete about what I'm suggesting:
>>
>> We'd have thread_struct.baseline_pkru.  It would start with key 0
>> allowing all access and key 1 denying reads.
>
> Are you sure thread_struct is the right place for this?  I think of
> signal handlers as a process-wide thing, and it seems a bit goofy if we
> have the PKRU value in a signal handler depend on the PKRU of the thread
> that got interrupted.

I think you're right.  mmu_context_t might be a better choice.

>
>> We'd have a syscall like set_protection_key that could allocate unused
>> keys and change the values of keys that have been allocated.  Those
>> changes would be reflected in baseline_pkru.  Changes to keys 0 and 1
>> in baseline_pkru would not be allowed.
>
> FWIW, I think we can do this without *actually* dedicating key 1 to
> execute-only.  But that's a side issue.
>
>> Signal delivery would load baseline_pkru into the PKRU register.
>> Signal restore would restore PKRU to its previous value.
>
> Do you really mean "its previous value" or are you OK with the existing
> behavior which restores PKRU from the XSAVE buffer in the sigcontext?

By "its previous value" I meant the value in the XSAVE buffer in the
sigcontext.  So I think I'm okay with that :)

>
>> WRPKRU would, of course, override baseline_pkru, but it wouldn't
>> change baseline_pkru.  The set_protection_key syscall would modify
>> *both* real PKRU and baseline_pkru.
>
> How about this:
>
> We make baseline_pkru a process-wide baseline and store it in
> mm->context.  That way, no matter which thread gets interrupted for a
> signal, they see consistent values.  We only write to it when an app
> _specifically_ asks for it to be updated with a special flag to
> sys_pkey_set().
>
> When an app uses the execute-only support, we implicitly set the
> read-disable bit in baseline_pkru for the execute-only pkey.

Sounds good, I think.

--Andy
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