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 <[email protected]> > > > > 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 <[email protected]> > > Signed-off-by: Borislav Petkov <[email protected]> > > Acked-by: Ingo Molnar <[email protected]> > > Acked-by: Thomas Gleixner <[email protected]> > > Cc: Andy Lutomirski <[email protected]> > > Cc: Dave Hansen <[email protected]> > > Cc: "H. Peter Anvin" <[email protected]> > > Cc: "Jason A. Donenfeld" <[email protected]> > > Cc: kvm ML <[email protected]> > > Cc: Paolo Bonzini <[email protected]> > > Cc: Radim Krčmář <[email protected]> > > Cc: Rik van Riel <[email protected]> > > Cc: [email protected] > > Cc: x86-ml <[email protected]> > > Link: http://lkml.kernel.org/r/[email protected] > > Link: https://lkml.kernel.org/r/[email protected] > > Signed-off-by: Sebastian Andrzej Siewior <[email protected]> > > Signed-off-by: Greg Kroah-Hartman <[email protected]> > > --- > > 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. thanks, greg k-h
