[PATCH] x86/fpu: Disable BH while while loading FPU registers in __fpu__restore_sig()

[Sebastian Andrzej Siewior]

The sequence
   fpu->initialized = 1;
   preempt_disable();
   fpu__restore(fpu);
   preempt_enable();

is racy in regard to a context switch. A context switch after the first
line would save the `actual' content of the FPU registers and trash away
the state that has been prepared (since fpu__drop()).

Use local_bh_disable() around the restore sequence to avoid the race. BH
needs to be disabled because BH is allowed to run (even with preemption
disabled) and might invoke kernel_fpu_begin().

This possible race has been reported by the Kernel Test Robot in FEB
2016 while there still was lazy FPU support.

Link: https://lkml.kernel.org/r/20160226074940.GA28911@xxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 arch/x86/kernel/fpu/signal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 61a949d84dfa5..d99a8ee9e185e 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -344,10 +344,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 			sanitize_restored_xstate(tsk, &env, xfeatures, fx_only);
 		}
 
+		local_bh_disable();
 		fpu->initialized = 1;
-		preempt_disable();
 		fpu__restore(fpu);
-		preempt_enable();
+		local_bh_enable();
 
 		return err;
 	} else {
-- 
2.19.1