[PATCH 4.14 028/107] powerpc/64: Use barrier_nospec in syscall entry

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

commit 51973a815c6b46d7b23b68d6af371ad1c9d503ca 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@xxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/powerpc/kernel/entry_64.S |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -36,6 +36,7 @@
 #include <asm/context_tracking.h>
 #include <asm/tm.h>
 #include <asm/ppc-opcode.h>
+#include <asm/barrier.h>
 #include <asm/export.h>
 #ifdef CONFIG_PPC_BOOK3S
 #include <asm/exception-64s.h>
@@ -179,6 +180,15 @@ system_call:			/* label this so stack tr
 	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 */