Re: [PATCH] x86/fpu: Correct pkru/xstate inconsistency

[Dave Hansen <dave.hansen@intel.com>]

On 2/15/22 09:50, Brian Geffon wrote:
>>> is not a kernel thread as kernel threads will never use PKRU. It's
>>> possible that this_cpu_read_stable() on current_task (ie.
>>> get_current()) is returning an old cached value. By forcing the read
>>> with this_cpu_read() the correct task is used. Without this it's
>>> possible when switching from a kernel thread to a userspace thread
>>> that we'll still observe the PF_KTHREAD flag and never restore the
>>> PKRU. And as a result this issue only occurs when switching from a
>>> kernel thread to a userspace thread, switching from a non kernel
>>> thread works perfectly fine because all we consider in that situation
>>> is the flags from some other non kernel task and the next fpu is
>>> passed in to switch_fpu_finish().
>>
>> It makes *sense* that there would be a place in the context switch code
>> where 'current' is wonky, but I never realized this.  This seems really
>> fragile, but *also* trivially detectable.
>>
>> Is the PKRU code really the only code to use 'current' in a buggy way
>> like this?
> 
> Yes, because the remaining code in __switch_to() references the next
> task as next_p rather than current. Technically, it might be more
> correct to pass next_p to switch_fpu_finish(), what do you think? This
> may make sense since we're also passing the next fpu anyway.

Yeah, passing next_p instead of next_fpu makes a lot of sense to me.