x86: fix fpu restore from sig return

If the task never used fpu, initialize the fpu before restoring the FP
state from the signal handler context. This will allocate the fpu
state, if the task never needed it before.
Reported-and-bisected-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Tested-by: Eric Sesterhenn <snakebyte@gmx.de>
Cc: Frederik Deweerdt <deweerdt@free.fr>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/i387.c

@@ -450,7 +450,6 @@ static inline int restore_i387_fsave(struct _fpstate_ia32 __user *buf)
 {
 	struct task_struct *tsk = current;
 
-	clear_fpu(tsk);
 	return __copy_from_user(&tsk->thread.xstate->fsave, buf,
 				sizeof(struct i387_fsave_struct));
 }
@@ -461,7 +460,6 @@ static int restore_i387_fxsave(struct _fpstate_ia32 __user *buf)
 	struct user_i387_ia32_struct env;
 	int err;
 
-	clear_fpu(tsk);
 	err = __copy_from_user(&tsk->thread.xstate->fxsave, &buf->_fxsr_env[0],
 			       sizeof(struct i387_fxsave_struct));
 	/* mxcsr reserved bits must be masked to zero for security reasons */
@@ -478,6 +476,16 @@ int restore_i387_ia32(struct _fpstate_ia32 __user *buf)
 	int err;
 
 	if (HAVE_HWFP) {
+		struct task_struct *tsk = current;
+
+		clear_fpu(tsk);
+
+		if (!used_math()) {
+			err = init_fpu(tsk);
+			if (err)
+				return err;
+		}
+
 		if (cpu_has_fxsr)
 			err = restore_i387_fxsave(buf);
 		else

include/asm-x86/i387.h

@@ -175,7 +175,15 @@ static inline int save_i387(struct _fpstate __user *buf)
  */
 static inline int restore_i387(struct _fpstate __user *buf)
 {
-	set_used_math();
+	struct task_struct *tsk = current;
+	int err;
+
+	if (!used_math()) {
+		err = init_fpu(tsk);
+		if (err)
+			return err;
+	}
+
 	if (!(task_thread_info(current)->status & TS_USEDFPU)) {
 		clts();
 		task_thread_info(current)->status |= TS_USEDFPU;