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


Greg Kroah-Hartman <gregkh@...>
 

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@...>

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@...>
Signed-off-by: Borislav Petkov <bp@...>
Acked-by: Ingo Molnar <mingo@...>
Acked-by: Thomas Gleixner <tglx@...>
Cc: Andy Lutomirski <luto@...>
Cc: Dave Hansen <dave.hansen@...>
Cc: "H. Peter Anvin" <hpa@...>
Cc: "Jason A. Donenfeld" <Jason@...>
Cc: kvm ML <kvm@...>
Cc: Paolo Bonzini <pbonzini@...>
Cc: Radim Krčmář <rkrcmar@...>
Cc: Rik van Riel <riel@...>
Cc: stable@...
Cc: x86-ml <x86@...>
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@...>
Signed-off-by: Greg Kroah-Hartman <gregkh@...>
---
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

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