x86/fpu: Split an fpstate_alloc_init() function out of init_fpu()

Most init_fpu() users don't want the register-saving aspect of the
function, they are calling it for 'current' and when FPU registers
are not allocated and initialized yet.

Split out a simplified API that does just that (and add debug-checks
for these conditions): fpstate_alloc_init().

Use it where appropriate.
Reviewed-by: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
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/i387.h

@@ -18,7 +18,10 @@
 struct pt_regs;
 struct user_i387_struct;
 
+extern int fpstate_alloc_init(struct task_struct *curr);
+
 extern int init_fpu(struct task_struct *child);
+
 extern void fpu_finit(struct fpu *fpu);
 extern int dump_fpu(struct pt_regs *, struct user_i387_struct *);
 extern void math_state_restore(void);

arch/x86/kernel/i387.c

@@ -246,6 +246,37 @@ void fpu_finit(struct fpu *fpu)
 }
 EXPORT_SYMBOL_GPL(fpu_finit);
 
+/*
+ * Allocate the backing store for the current task's FPU registers
+ * and initialize the registers themselves as well.
+ *
+ * Can fail.
+ */
+int fpstate_alloc_init(struct task_struct *curr)
+{
+	int ret;
+
+	if (WARN_ON_ONCE(curr != current))
+		return -EINVAL;
+	if (WARN_ON_ONCE(curr->flags & PF_USED_MATH))
+		return -EINVAL;
+
+	/*
+	 * Memory allocation at the first usage of the FPU and other state.
+	 */
+	ret = fpu_alloc(&curr->thread.fpu);
+	if (ret)
+		return ret;
+
+	fpu_finit(&curr->thread.fpu);
+
+	/* Safe to do for the current task: */
+	curr->flags |= PF_USED_MATH;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fpstate_alloc_init);
+
 /*
  * The _current_ task is using the FPU for the first time
  * so initialize it and set the mxcsr to its default

arch/x86/kernel/process.c

@@ -159,7 +159,7 @@ void flush_thread(void)
 	} else {
 		if (!tsk_used_math(tsk)) {
 			/* kthread execs. TODO: cleanup this horror. */
-			if (WARN_ON(init_fpu(tsk)))
+		if (WARN_ON(fpstate_alloc_init(tsk)))
 				force_sig(SIGKILL, tsk);
 			user_fpu_begin();
 		}

arch/x86/kernel/traps.c

@@ -846,7 +846,7 @@ void math_state_restore(void)
 		/*
 		 * does a slab alloc which can sleep
 		 */
-		if (init_fpu(tsk)) {
+		if (fpstate_alloc_init(tsk)) {
 			/*
 			 * ran out of memory!
 			 */

arch/x86/kernel/xsave.c

@@ -349,7 +349,7 @@ int __restore_xstate_sig(void __user *buf, void __user *buf_fx, int size)
 	if (!access_ok(VERIFY_READ, buf, size))
 		return -EACCES;
 
-	if (!used_math() && init_fpu(tsk))
+	if (!used_math() && fpstate_alloc_init(tsk))
 		return -1;
 
 	if (!static_cpu_has(X86_FEATURE_FPU))

arch/x86/kvm/x86.c

@@ -6600,7 +6600,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	int r;
 	sigset_t sigsaved;
 
-	if (!tsk_used_math(current) && init_fpu(current))
+	if (!tsk_used_math(current) && fpstate_alloc_init(current))
 		return -ENOMEM;
 
 	if (vcpu->sigset_active)

arch/x86/math-emu/fpu_entry.c

@@ -149,7 +149,7 @@ void math_emulate(struct math_emu_info *info)
 	struct desc_struct code_descriptor;
 
 	if (!used_math()) {
-		if (init_fpu(current)) {
+		if (fpstate_alloc_init(current)) {
 			do_group_exit(SIGKILL);
 			return;
 		}