Messages in this thread | | | Subject | Re: [PATCH 4.9 18/22] x86/fpu: Disable bottom halves while loading FPU registers | From | Jan Kiszka <> | Date | Fri, 24 Jul 2020 19:07:06 +0200 |
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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
-- Siemens AG, Corporate Technology, CT RDA IOT SES-DE Corporate Competence Center Embedded Linux
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