x86/fpu: Fix FPU register read access to the current task

Bobby Powers reported the following FPU warning during ELF coredumping:

   WARNING: CPU: 0 PID: 27452 at arch/x86/kernel/fpu/core.c:324 fpu__activate_stopped+0x8a/0xa0()

This warning unearthed an invalid assumption about fpu__activate_stopped()
that I added in:

  67e97fc2ec57 ("x86/fpu: Rename init_fpu() to fpu__unlazy_stopped() and add debugging check")

the old init_fpu() function had an (intentional but obscure) side effect:
when FPU registers are accessed for the current task, for reading, then
it synchronized live in-register FPU state with the fpstate by saving it.

So fix this bug by saving the FPU if we are the current task. We'll
still warn in fpu__save() if this is called for not yet stopped
child tasks, so the debugging check is still preserved.

Also rename the function to fpu__activate_fpstate(), because it's not
exclusively used for stopped tasks, but for the current task as well.

( Note that this bug calls for a cleaner separation of access-for-read
  and access-for-modification FPU methods, but we'll do that in separate
  patches. )

Reported-by: Bobby Powers <bobbypowers@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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

@@ -22,7 +22,7 @@
  * High level FPU state handling functions:
  */
 extern void fpu__activate_curr(struct fpu *fpu);
-extern void fpu__activate_stopped(struct fpu *fpu);
+extern void fpu__activate_fpstate(struct fpu *fpu);
 extern void fpu__save(struct fpu *fpu);
 extern void fpu__restore(struct fpu *fpu);
 extern int  fpu__restore_sig(void __user *buf, int ia32_frame);

arch/x86/kernel/fpu/core.c

@@ -296,40 +296,47 @@ void fpu__activate_curr(struct fpu *fpu)
 EXPORT_SYMBOL_GPL(fpu__activate_curr);
 
 /*
- * This function must be called before we modify a stopped child's
+ * This function must be called before we read or write a task's fpstate.
+ *
+ * If the task has not used the FPU before then initialize its
  * fpstate.
  *
- * If the child has not used the FPU before then initialize its
- * fpstate.
+ * If the task has used the FPU before then save and unlazy it.
  *
- * If the child has used the FPU before then unlazy it.
- *
- * [ After this function call, after registers in the fpstate are
+ * [ If this function is used for non-current child tasks, then
+ *   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 lazy status here then the lazy in-registers
  *   state pending on its former CPU could be restored, corrupting
- *   the modifications. ]
+ *   the modifications.
  *
- * This function is also called before we read a stopped child's
- * FPU state - to make sure it's initialized if the child has
- * no active FPU state.
+ *   This function can be used for the current task as well, but
+ *   only for reading the fpstate. Modifications to the fpstate
+ *   will be lost on eagerfpu systems. ]
  *
  * TODO: A future optimization would be to skip the unlazying in
  *       the read-only case, it's not strictly necessary for
  *       read-only access to the context.
  */
-void fpu__activate_stopped(struct fpu *child_fpu)
+void fpu__activate_fpstate(struct fpu *fpu)
 {
-	WARN_ON_FPU(child_fpu == &current->thread.fpu);
-
-	if (child_fpu->fpstate_active) {
-		child_fpu->last_cpu = -1;
+	/*
+	 * If fpregs are active (in the current CPU), then
+	 * copy them to the fpstate:
+	 */
+	if (fpu->fpregs_active) {
+		fpu__save(fpu);
 	} else {
-		fpstate_init(&child_fpu->state);
+		if (fpu->fpstate_active) {
+			/* Invalidate any lazy state: */
+			fpu->last_cpu = -1;
+		} else {
+			fpstate_init(&fpu->state);
 
-		/* Safe to do for stopped child tasks: */
-		child_fpu->fpstate_active = 1;
+			/* Safe to do for current and for stopped child tasks: */
+			fpu->fpstate_active = 1;
+		}
 	}
 }
 

arch/x86/kernel/fpu/regset.c

@@ -33,7 +33,7 @@ int xfpregs_get(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_has_fxsr)
 		return -ENODEV;
 
-	fpu__activate_stopped(fpu);
+	fpu__activate_fpstate(fpu);
 	fpstate_sanitize_xstate(fpu);
 
 	return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
@@ -50,7 +50,7 @@ int xfpregs_set(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_has_fxsr)
 		return -ENODEV;
 
-	fpu__activate_stopped(fpu);
+	fpu__activate_fpstate(fpu);
 	fpstate_sanitize_xstate(fpu);
 
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
@@ -82,7 +82,7 @@ int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_has_xsave)
 		return -ENODEV;
 
-	fpu__activate_stopped(fpu);
+	fpu__activate_fpstate(fpu);
 
 	xsave = &fpu->state.xsave;
 
@@ -111,7 +111,7 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_has_xsave)
 		return -ENODEV;
 
-	fpu__activate_stopped(fpu);
+	fpu__activate_fpstate(fpu);
 
 	xsave = &fpu->state.xsave;
 
@@ -273,7 +273,7 @@ int fpregs_get(struct task_struct *target, const struct user_regset *regset,
 	struct fpu *fpu = &target->thread.fpu;
 	struct user_i387_ia32_struct env;
 
-	fpu__activate_stopped(fpu);
+	fpu__activate_fpstate(fpu);
 
 	if (!static_cpu_has(X86_FEATURE_FPU))
 		return fpregs_soft_get(target, regset, pos, count, kbuf, ubuf);
@@ -303,7 +303,7 @@ int fpregs_set(struct task_struct *target, const struct user_regset *regset,
 	struct user_i387_ia32_struct env;
 	int ret;
 
-	fpu__activate_stopped(fpu);
+	fpu__activate_fpstate(fpu);
 	fpstate_sanitize_xstate(fpu);
 
 	if (!static_cpu_has(X86_FEATURE_FPU))