Messages in this thread | | | Date | Thu, 10 Jun 2021 10:10:51 -0700 | From | "Andy Lutomirski" <> | Subject | Re: [patch V3 3/6] x86/process: Check PF KTHREAD and not cur rent->mm for kernel threads |
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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.
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