Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

[Andy Lutomirski]

> On Oct 18, 2018, at 9:26 AM, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:
> 
>> On 2018-10-12 11:02:18 [-0700], Andy Lutomirski wrote:
>> On Fri, Oct 12, 2018 at 10:54 AM Dave Hansen
>> <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>>> 
>>>> On 10/04/2018 07:05 AM, Sebastian Andrzej Siewior wrote:
>>>> The PKRU value is not set for kernel threads because they do not have
>>>> the ->initialized value set. As a result the kernel thread has a random
>>>> PKRU value set which it inherits from the previous task.
>>>> It has been suggested by Paolo Bonzini to set it for kernel threads, too
>>>> because it might be a fix.
>>>> I *think* this is not required because the kernel threads don't copy
>>>> data to/from userland and don't have access to any userspace mm in
>>>> general.
>>>> However there is this use_mm(). If we gain a mm by use_mm() we don't
>>>> have a matching PKRU value because those are per thread. It has been
>>>> suggested to use 0 as the PKRU value but this would bypass PKRU.
>>>> 
>>>> Set the initial (default) PKRU value for kernel threads.
>>> 
>>> We might want to do this for cleanliness reasons...  Maybe.
>>> 
>>> But this *should* have no practical effects.  Kernel threads have no
>>> real 'mm' and no user pages.  They should not have do access to user
>>> mappings.  Protection keys *only* apply to user mappings.  Thus,
>>> logically, they should never be affected by PKRU values.
>>> 
>>> So I'm kinda missing the point of the patch.
>> 
>> use_mm().
> 
> So. I would drop that patch from queue. Anyone feels different about it?
> 

I think we *do* want the patch. Itâs a bugfix for use_mm users, right?

> Sebastian