powerpc/64: Use barrier_nospec in syscall entry

BugLink: https://bugs.launchpad.net/bugs/1830176

commit 51973a81 upstream.

Our syscall entry is done in assembly so patch in an explicit
barrier_nospec.

Based on a patch by Michal Suchanek.
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/powerpc/kernel/entry_64.S

@@ -36,6 +36,7 @@
 #include <asm/hw_irq.h>
 #include <asm/context_tracking.h>
 #include <asm/tm.h>
+#include <asm/barrier.h>
 #ifdef CONFIG_PPC_BOOK3S
 #include <asm/exception-64s.h>
 #else
@@ -177,6 +178,15 @@ system_call:			/* label this so stack traces look sane */
 	clrldi	r8,r8,32
 15:
 	slwi	r0,r0,4
+
+	barrier_nospec_asm
+	/*
+	 * Prevent the load of the handler below (based on the user-passed
+	 * system call number) being speculatively executed until the test
+	 * against NR_syscalls and branch to .Lsyscall_enosys above has
+	 * committed.
+	 */
+
 	ldx	r12,r11,r0	/* Fetch system call handler [ptr] */
 	mtctr   r12
 	bctrl			/* Call handler */