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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Thu, 6 Jun 2024 10:48:33 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org,
	Andy Lutomirski <luto@amacapital.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave@sr71.net>, Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH 3/3, v4] x86/fpu: Remove init_task FPU state
 dependencies, add debugging warning for PF_KTHREAD tasks
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* Ingo Molnar <mingo@kernel.org> wrote:

> I changed the debug check to test for PF_KTHREAD, and to return NULL:
> 
> +#ifdef CONFIG_X86_DEBUG_FPU
> +struct fpu *x86_task_fpu(struct task_struct *task)
> +{
> +	if (WARN_ON_ONCE(task->flags & PF_KTHREAD))
> +		return NULL;
> +
> +	return (void *)task + sizeof(*task);
> +}
> +#endif
> 
> ... and the NULL we return will likely crash & exit any kthreads attempting 
> to use the FPU context area - which I think is preferable to returning 
> invalid memory that may then be corrupted.
> 
> Hopefully this remains a hypothethical concern. :-)
> 
> Alternatively, this may be one of the very few cases where a BUG_ON() might 
> be justified? This condition is not recoverable in any sane fashion IMO.

And promptly this triggered in live testing, because while kthreads do not 
use the FPU context area, the current fpu__drop() code does call 
x86_task_cpu() even for kthreads...

See the two new 4/3 and 5/3 patches in this thread I've sent that clean 
this up:

  [PATCH 4/3] x86/fpu: Push 'fpu' pointer calculation into the fpu__drop() call
  [PATCH 5/3] x86/fpu: Make sure x86_task_fpu() doesn't get called for PF_KTHREAD tasks during exit

I'll also reorder the patches to apply these fpu__drop() changes before 
adding the debug warning.

Thanks,

	Ingo