Messages in this thread | | | Date | Fri, 24 Jul 2020 14:12:02 -0400 | From | Sasha Levin <> | Subject | Re: [PATCH 4.9 18/22] x86/fpu: Disable bottom halves while loading FPU registers |
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On Fri, Jul 24, 2020 at 07:44:37PM +0200, Greg Kroah-Hartman wrote: >On Fri, Jul 24, 2020 at 07:07:06PM +0200, Jan Kiszka wrote: >> 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). > >You are asking about something we did back in 2018. I can't remember >what I did last week :) > >If you provide a backport that works, I'll be glad to take it. The >current patch does not apply cleanly there at all.
The conflict was due to a missing rename back in 4.4: e4a81bfcaae1 ("x86/fpu: Rename fpu::fpstate_active to fpu::initialized").
I've fixed up the patch and queued it for 4.4, thanks for pointing it out Jan!
-- Thanks, Sasha
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