Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 21 Dec 2015 15:07:19 -0800 | Subject | Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit? |
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On Mon, Dec 21, 2015 at 3:04 PM, Dave Hansen <dave.hansen@linux.intel.com> 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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