lkml.org 
[lkml]   [2021]   [Jun]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [patch V3 3/6] x86/process: Check PF_KTHREAD and not current->mm for kernel threads
From
Date
On 6/10/21 1:54 PM, Thomas Gleixner wrote:
> On Thu, Jun 10 2021 at 10:10, Andy Lutomirski wrote:
>
>> On Tue, Jun 8, 2021, at 7:36 AM, Thomas Gleixner wrote:
>>> switch_fpu_finish() checks current->mm as indicator for kernel threads.
>>> That's wrong because kernel threads can temporarily use a mm of a user
>>> process via kthread_use_mm().
>>>
>>> Check the task flags for PF_KTHREAD instead.
>>>
>>> Fixes: 0cecca9d03c9 ("x86/fpu: Eager switch PKRU state")
>>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>>> Cc: Rik van Riel <riel@surriel.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>> arch/x86/include/asm/fpu/internal.h | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> --- a/arch/x86/include/asm/fpu/internal.h
>>> +++ b/arch/x86/include/asm/fpu/internal.h
>>> @@ -578,7 +578,7 @@ static inline void switch_fpu_finish(str
>>> * PKRU state is switched eagerly because it needs to be valid before we
>>> * return to userland e.g. for a copy_to_user() operation.
>>> */
>>> - if (current->mm) {
>>> + if (!(current->flags & PF_KTHREAD)) {
>>> pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
>>> if (pk)
>>> pkru_val = pk->pkru;
>>>
>>>
>> Why are we checking this at all? I actually tend to agree with the
>> ->mm check more than PF_anything. If we have a user address space,
>> then PKRU matters. If we don’t, then it doesn’t.
>
> Which PKRU matters? A kernel thread has always the default PKRU no
> matter whether it uses a mm or not. It _cannot_ borrow the PKRU from the
> mm owning process. There is no way, so let's not pretend there would be.
>

Hmm. I guess PK_KTHREAD is consistent with switch_fpu_prepare() --
kernel threads have no FPU state.

It might be worth a loud comment here that kernel threads' PKRU is not
context switched and that, if anyone wants kthread_use_mm() users to use
anything other than the default PKRU, that they will need to change this.

So I guess your patch is okay.

--Andy

\
 
 \ /
  Last update: 2021-06-11 03:05    [W:0.147 / U:0.200 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site