Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs

[Oleg Nesterov]

On 03/06, Oleg Nesterov wrote:
>
> OK, but please note that this patch is not beckportable. If you think
> that -stable doesn't need this fix, then I agree.
>
> If the caller is do_device_not_available(), then we can not enable
> irqs before __thread_fpu_begin() + restore_fpu_checking().
>
> 1. Preemption in between can destroy ->fpu.state initialized by
>    fpu_finit(), __switch_to() will save the live (wrong) FPU state
>    again.
>
> 2. kernel_fpu_begin() from irq right after __thread_fpu_begin() is
>    not nice too. It will do __save_init_fpu() and this overwrites
>    ->fpu.state too.
>
> Starting from v4.0 it does kernel_fpu_disable(), but the older kernels
> do not.
>
> Ingo, this code is really horrible and fragile. We need to cleanup it
> step-by-step, imho.

Forgot to mention...

And, otoh, if we are not going to backport this change, then I think this
irq_enable() should be called by do_device_not_available().

>
> > ======================>
> > From: Ingo Molnar <mingo@xxxxxxxxxx>
> > Date: Fri, 6 Mar 2015 08:37:57 +0100
> > Subject: [PATCH] x86/fpu: Don't disable irqs in math_state_restore()
> >
> > math_state_restore() was historically called with irqs disabled,
> > because that's how the hardware generates the trap, and also because
> > back in the days it was possible for it to be an asynchronous
> > interrupt and interrupt handlers run with irqs off.
> >
> > These days it's always an instruction trap, and furthermore it does
> > inevitably complex things such as memory allocation and signal
> > processing, which is not done with irqs disabled.
> >
> > So keep irqs enabled.
> >
> > This might surprise in-kernel FPU users that somehow relied on
> > interrupts being disabled across FPU usage - but that's
> > fundamentally fragile anyway due to the inatomicity of FPU state
> > restores. The trap return will restore interrupts to its previous
> > state, but if FPU ops trigger math_state_restore() there's no
> > guarantee of atomicity anymore.
> >
> > To warn about in-kernel irqs-off users of FPU state we might want to
> > pass 'struct pt_regs' to math_state_restore() and check the trapped
> > state for irqs disabled (flags has IF cleared) and kernel context -
> > but that's for a later patch.
> >
> > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > Cc: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> > ---
> >  arch/x86/kernel/traps.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index 950815a138e1..52f9e4057cee 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -844,8 +844,9 @@ void math_state_restore(void)
> >  {
> >  	struct task_struct *tsk = current;
> >
> > +	local_irq_enable();
> > +
> >  	if (!tsk_used_math(tsk)) {
> > -		local_irq_enable();
> >  		/*
> >  		 * does a slab alloc which can sleep
> >  		 */
> > @@ -856,7 +857,6 @@ void math_state_restore(void)
> >  			do_group_exit(SIGKILL);
> >  			return;
> >  		}
> > -		local_irq_disable();
> >  	}
> >
> >  	/* Avoid __kernel_fpu_begin() right after __thread_fpu_begin() */

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