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 10:10:51 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Thomas Gleixner" <tglx@...utronix.de>,
        "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 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.