Re: [PATCH 4.9 18/22] x86/fpu: Disable bottom halves while loading FPU registers

Jan Kiszka
2020-07-24T17:07:47Z

On 28.12.18 12:52, Greg Kroah-Hartman wrote:
> 4.9-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>
> commit 68239654acafe6aad5a3c1dc7237e60accfebc03 upstream.
>
> The sequence
>
> fpu->initialized = 1; /* step A */
> preempt_disable(); /* step B */
> fpu__restore(fpu);
> preempt_enable();
>
> in __fpu__restore_sig() 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->initialized to 0.
>
> After fpu->initialized is cleared, the CPU's FPU state is not saved
> to fpu->state during a context switch. The new state is loaded via
> fpu__restore(). It gets loaded into fpu->state from userland and
> ensured it is sane. fpu->initialized is then set to 1 in order to avoid
> fpu__initialize() doing anything (overwrite the new state) which is part
> of fpu__restore().
>
> A context switch between step A and B above would save CPU's current FPU
> registers to fpu->state and overwrite the newly prepared state. This
> looks like a 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.
>
> Disable bottom halves around the restore sequence to avoid the race. BH
> need to be disabled because BH is allowed to run (even with preemption
> disabled) and might invoke kernel_fpu_begin() by doing IPsec.
>
> [ bp: massage commit message a bit. ]
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> Acked-by: Ingo Molnar <mingo@kernel.org>
> Acked-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
> Cc: kvm ML <kvm@vger.kernel.org>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: stable@vger.kernel.org
> Cc: x86-ml <x86@kernel.org>
> Link: http://lkml.kernel.org/r/20181120102635.ddv3fvavxajjlfqk@linutronix.de
> Link: https://lkml.kernel.org/r/20160226074940.GA28911@pd.tnic
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> arch/x86/kernel/fpu/signal.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -342,10 +342,10 @@ static int __fpu__restore_sig(void __use
> sanitize_restored_xstate(tsk, &env, xfeatures, fx_only);
> }
>
> + local_bh_disable();
> fpu->fpstate_active = 1;
> - preempt_disable();
> fpu__restore(fpu);
> - preempt_enable();
> + local_bh_enable();
>
> return err;
> } else {
>
>

Any reason why the backport stopped back than at 4.9? I just debugged
this out of a 4.4 kernel, and it is needed there as well. I'm happy to
propose a backport, would just appreciate a hint if the BH protection is
needed also there (my case was without BH).

Thanks,
Jan

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