x86/fpu: Move fpu__write_begin() to regset

The only usecase for fpu__write_begin is the set() callback of regset, so
the function is pointlessly global.

Move it to the regset code and rename it to fpu_force_restore() which is
exactly decribing what the function does.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121453.328652975@linutronix.de

arch/x86/include/asm/fpu/internal.h

@@ -26,7 +26,6 @@
 /*
  * High level FPU state handling functions:
  */
-extern void fpu__prepare_write(struct fpu *fpu);
 extern void fpu__save(struct fpu *fpu);
 extern int  fpu__restore_sig(void __user *buf, int ia32_frame);
 extern void fpu__drop(struct fpu *fpu);

arch/x86/kernel/fpu/core.c

@@ -281,30 +281,6 @@ static void fpu__initialize(struct fpu *fpu)
 	trace_x86_fpu_init_state(fpu);
 }
 
-/*
- * This function must be called before we write a task's fpstate.
- *
- * Invalidate any cached FPU registers.
- *
- * After this function call, after registers in the fpstate are
- * modified and the child task has woken up, the child task will
- * restore the modified FPU state from the modified context. If we
- * didn't clear its cached status here then the cached in-registers
- * state pending on its former CPU could be restored, corrupting
- * the modifications.
- */
-void fpu__prepare_write(struct fpu *fpu)
-{
-	/*
-	 * Only stopped child tasks can be used to modify the FPU
-	 * state in the fpstate buffer:
-	 */
-	WARN_ON_FPU(fpu == &current->thread.fpu);
-
-	/* Invalidate any cached state: */
-	__fpu_invalidate_fpregs_state(fpu);
-}
-
 /*
  * Drops current FPU state: deactivates the fpregs and
  * the fpstate. NOTE: it still leaves previous contents

arch/x86/kernel/fpu/regset.c

@@ -44,6 +44,25 @@ static void sync_fpstate(struct fpu *fpu)
 		fpu__save(fpu);
 }
 
+/*
+ * Invalidate cached FPU registers before modifying the stopped target
+ * task's fpstate.
+ *
+ * This forces the target task on resume to restore the FPU registers from
+ * modified fpstate. Otherwise the task might skip the restore and operate
+ * with the cached FPU registers which discards the modifications.
+ */
+static void fpu_force_restore(struct fpu *fpu)
+{
+	/*
+	 * Only stopped child tasks can be used to modify the FPU
+	 * state in the fpstate buffer:
+	 */
+	WARN_ON_FPU(fpu == &current->thread.fpu);
+
+	__fpu_invalidate_fpregs_state(fpu);
+}
+
 int xfpregs_get(struct task_struct *target, const struct user_regset *regset,
 		struct membuf to)
 {
@@ -88,7 +107,7 @@ int xfpregs_set(struct task_struct *target, const struct user_regset *regset,
 	if (newstate.mxcsr & ~mxcsr_feature_mask)
 		return -EINVAL;
 
-	fpu__prepare_write(fpu);
+	fpu_force_restore(fpu);
 
 	/* Copy the state  */
 	memcpy(&fpu->state.fxsave, &newstate, sizeof(newstate));
@@ -146,7 +165,7 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 		}
 	}
 
-	fpu__prepare_write(fpu);
+	fpu_force_restore(fpu);
 	ret = copy_kernel_to_xstate(&fpu->state.xsave, kbuf ?: tmpbuf);
 
 out:
@@ -346,7 +365,7 @@ int fpregs_set(struct task_struct *target, const struct user_regset *regset,
 	if (ret)
 		return ret;
 
-	fpu__prepare_write(fpu);
+	fpu_force_restore(fpu);
 
 	if (cpu_feature_enabled(X86_FEATURE_FXSR))
 		convert_to_fxsr(&fpu->state.fxsave, &env);