Re: [PATCH v2] x86/fpu: Disable BH while while loading FPU registers in __fpu__restore_sig()

[Borislav Petkov]

On Mon, Nov 19, 2018 at 05:04:10PM +0100, Sebastian Andrzej Siewior wrote:
> The sequence
>    fpu->initialized = 1; /* step A */
>    preempt_disable();	 /* step B */
>    fpu__restore(fpu);
>    preempt_enable();
> 
> is racy in regard to a context switch.
> For 32bit frames __fpu__restore_sig() prepares the FPU state within
> fpu->state. To ensure that a context switch (switch_fpu_prepare() in
> particular) does not modify fpu->state it uses fpu__drop() which sets
> fpu->initializes to 0.

"... ->initialized to 0."

Also, a new line here pls.

> With this change the CPU's FPU state is not saved
		  ^

comma:

		 ,

Also, instead of "with this change" I think you mean: "After
->initialized is cleared, the CPU's FPU state..."

> to fpu->state during a context switch.
> It then loads the state to fpu->state from userland and ensures it
> sane.

"... and ensures it is sane."

> The new state is loaded via fpu__restore(). The code sets then
> fpu->initializes to 1 in order to avoid fpu__initialize() doing

fpu->initialized

> anything (overwrite the new state) which is part of fpu__restore().

<---- newline here.

> A context switch between step A and B would save CPU's current FPU
> registers to fpu->state and overwrite the newly prepared state. This
> looks like tiny race window but the Kernel Test Robot reported this back
> in 2016 while we had lazy FPU support. Borislav Petkov made the link
> between that report and another patch that has been posted.
> Since the removal of the lazy FPU support, this race goes unnoticed
> because the warning has been removed.
> 
> Use local_bh_disable() around the restore sequence to avoid the race. BH

Let's write it out once: "Bottom halves need to be... "

> needs to be disabled because BH is allowed to run (even with preemption
> disabled) and might invoke kernel_fpu_begin().

... and let's put the potential example here with IPsec and softirq.

> Link: https://lkml.kernel.org/r/20160226074940.GA28911@xxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> v1âv2: A more verbose commit as message.

Very much needed, thanks!

>  arch/x86/kernel/fpu/signal.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 61a949d84dfa5..d99a8ee9e185e 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -344,10 +344,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  			sanitize_restored_xstate(tsk, &env, xfeatures, fx_only);
>  		}
>  
> +		local_bh_disable();
>  		fpu->initialized = 1;
> -		preempt_disable();
>  		fpu__restore(fpu);
> -		preempt_enable();
> +		local_bh_enable();
>  
>  		return err;
>  	} else {
> -- 

-- 
Regards/Gruss,
    Boris.

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