linux-kernel - Re: [patch V3 3/6] x86/process: Check PF_KTHREAD and not current->mm for kernel threads

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Date:   Thu, 10 Jun 2021 22:54:24 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Rik van Riel <riel@...riel.com>, Borislav Petkov <bp@...e.de>
Subject: Re: [patch V3 3/6] x86/process: Check PF_KTHREAD and not current->mm for kernel threads

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@...utronix.de>
>> Cc: Rik van Riel <riel@...riel.com>
>> Cc: stable@...r.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