powerpc/8xx: Add Kernel Userspace Execution Prevention

This patch adds Kernel Userspace Execution Prevention on the 8xx.

When a page is Executable, it is set Executable for Key 0 and NX
for Key 1.

Up to now, the User group is defined with Key 0 for both User and
Supervisor.

By changing the group to Key 0 for User and Key 1 for Supervisor,
this patch prevents the Kernel from being able to execute user code.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/nohash/32/mmu-8xx.h

@@ -41,6 +41,13 @@
  */
 #define MI_APG_INIT	0x4fffffff
 
+/*
+ * 0 => Kernel => 01 (all accesses performed according to page definition)
+ * 1 => User => 10 (all accesses performed according to swaped page definition)
+ * 2-16 => NA => 11 (all accesses performed as user iaw page definition)
+ */
+#define MI_APG_KUEP	0x6fffffff
+
 /* The effective page number register.  When read, contains the information
  * about the last instruction TLB miss.  When MI_RPN is written, bits in
  * this register are used to create the TLB entry.

arch/powerpc/mm/8xx_mmu.c

@@ -213,3 +213,15 @@ void flush_instruction_cache(void)
 	mtspr(SPRN_IC_CST, IDC_INVALL);
 	isync();
 }
+
+#ifdef CONFIG_PPC_KUEP
+void __init setup_kuep(bool disabled)
+{
+	if (disabled)
+		return;
+
+	pr_info("Activating Kernel Userspace Execution Prevention\n");
+
+	mtspr(SPRN_MI_AP, MI_APG_KUEP);
+}
+#endif

arch/powerpc/platforms/Kconfig.cputype

@@ -34,6 +34,7 @@ config PPC_8xx
 	bool "Freescale 8xx"
 	select FSL_SOC
 	select SYS_SUPPORTS_HUGETLBFS
+	select PPC_HAVE_KUEP
 
 config 40x
 	bool "AMCC 40x"