Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [patch V3 3/6] x86/process: Check PF_KTHREAD and not current->mm for kernel threads | Date | Thu, 10 Jun 2021 22:54:24 +0200 |
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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.
Thanks,
tglx
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