[RFC PATCH v1 4/6] powerpc/8xx: Add Kernel Userspace Execution Prevention

[Christophe Leroy]

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@xxxxxx>
---
 arch/powerpc/include/asm/mmu-8xx.h     |  6 ++++++
 arch/powerpc/mm/8xx_mmu.c              | 10 ++++++++++
 arch/powerpc/platforms/Kconfig.cputype |  1 +
 3 files changed, 17 insertions(+)

diff --git a/arch/powerpc/include/asm/mmu-8xx.h b/arch/powerpc/include/asm/mmu-8xx.h
index fa05aa566ece..53dbf0788fce 100644
--- a/arch/powerpc/include/asm/mmu-8xx.h
+++ b/arch/powerpc/include/asm/mmu-8xx.h
@@ -41,6 +41,12 @@
  */
 #define MI_APG_INIT	0x44444444
 
+/*
+ * 0 => No user => 01 (all accesses performed according to page definition)
+ * 1 => User => 10 (all accesses performed according to swaped page definition)
+ */
+#define MI_APG_KUEP	0x66666666
+
 /* 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.
diff --git a/arch/powerpc/mm/8xx_mmu.c b/arch/powerpc/mm/8xx_mmu.c
index 01b7f5107c3a..597d3bd2d9b5 100644
--- a/arch/powerpc/mm/8xx_mmu.c
+++ b/arch/powerpc/mm/8xx_mmu.c
@@ -194,3 +194,13 @@ void flush_instruction_cache(void)
 	mtspr(SPRN_IC_CST, IDC_INVALL);
 	isync();
 }
+
+void setup_kuep(bool disabled)
+{
+	if (disabled)
+		return;
+
+	pr_warn("Activating Kernel Userspace Execution Prevention\n");
+
+	mtspr(SPRN_MI_AP, MI_APG_KUEP);
+}
diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index 70830cb3c18a..bbcae320324c 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -33,6 +33,7 @@ config PPC_8xx
 	bool "Freescale 8xx"
 	select FSL_SOC
 	select SYS_SUPPORTS_HUGETLBFS
+	select PPC_HAVE_KUEP
 
 config 40x
 	bool "AMCC 40x"
-- 
2.13.3