x86/fpu: Add a fastpath to copy_fpstate_to_sigframe()

Try to save the FPU registers directly to the userland stack frame if
the CPU holds the FPU registers for the current task. This has to be
done with the pagefault disabled because we can't fault (while the FPU
registers are locked) and therefore the operation might fail. If it
fails try the slowpath which can handle faults.

 [ bp: Massage a bit. ]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: kvm ML <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190403164156.19645-26-bigeasy@linutronix.de

arch/x86/kernel/fpu/signal.c

@@ -144,8 +144,10 @@ static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf)
  *	buf == buf_fx for 64-bit frames and 32-bit fsave frame.
  *	buf != buf_fx for 32-bit frames with fxstate.
  *
- * Save the state to task's fpu->state and then copy it to the user frame
- * pointed to by the aligned pointer 'buf_fx'.
+ * Try to save it directly to the user frame with disabled page fault handler.
+ * If this fails then do the slow path where the FPU state is first saved to
+ * task's fpu->state and then copy it to the user frame pointed to by the
+ * aligned pointer 'buf_fx'.
  *
  * If this is a 32-bit frame with fxstate, put a fsave header before
  * the aligned state at 'buf_fx'.
@@ -159,6 +161,7 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 	struct xregs_state *xsave = &fpu->state.xsave;
 	struct task_struct *tsk = current;
 	int ia32_fxstate = (buf != buf_fx);
+	int ret = -EFAULT;
 
 	ia32_fxstate &= (IS_ENABLED(CONFIG_X86_32) ||
 			 IS_ENABLED(CONFIG_IA32_EMULATION));
@@ -173,16 +176,22 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 
 	/*
 	 * If we do not need to load the FPU registers at return to userspace
-	 * then the CPU has the current state and we need to save it. Otherwise,
-	 * it has already been done and we can skip it.
+	 * then the CPU has the current state. Try to save it directly to
+	 * userland's stack frame if it does not cause a pagefault. If it does,
+	 * try the slowpath.
 	 */
 	fpregs_lock();
 	if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
+		pagefault_disable();
+		ret = copy_fpregs_to_sigframe(buf_fx);
+		pagefault_enable();
+		if (ret)
 			copy_fpregs_to_fpstate(fpu);
 		set_thread_flag(TIF_NEED_FPU_LOAD);
 	}
 	fpregs_unlock();
 
+	if (ret) {
 		if (using_compacted_format()) {
 			if (copy_xstate_to_user(buf_fx, xsave, 0, size))
 				return -1;
@@ -191,6 +200,7 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 			if (__copy_to_user(buf_fx, xsave, fpu_user_xstate_size))
 				return -1;
 		}
+	}
 
 	/* Save the fsave header for the 32-bit frames. */
 	if ((ia32_fxstate || !use_fxsr()) && save_fsave_header(tsk, buf))