powerpc/kuap: Wire-up KUAP on 40x

This adds KUAP support to 40x. This is done by checking
the content of SPRN_PID at the time user pgtable is loaded.

40x doesn't have KUEP, but KUAP implies KUEP because when the
PID doesn't match the page's PID, the page cannot be read nor
executed.

So KUEP is now automatically selected when KUAP is selected and
disabled when KUAP is disabled.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aaefa91897ddc42ac11019dc0e1d1a525bd08e90.1634627931.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_40x.S

@@ -298,6 +298,10 @@ _ASM_NOKPROBE_SYMBOL(\name\()_virt)
 3:
 	mfspr	r11,SPRN_SPRG_THREAD
 	lwz	r11,PGDIR(r11)
+#ifdef CONFIG_PPC_KUAP
+	rlwinm.	r9, r9, 0, 0xff
+	beq	5f			/* Kuap fault */
+#endif
 4:
 	tophys(r11, r11)
 	rlwimi	r11, r10, 12, 20, 29	/* Create L1 (pgdir/pmd) address */
@@ -378,6 +382,10 @@ _ASM_NOKPROBE_SYMBOL(\name\()_virt)
 3:
 	mfspr	r11,SPRN_SPRG_THREAD
 	lwz	r11,PGDIR(r11)
+#ifdef CONFIG_PPC_KUAP
+	rlwinm.	r9, r9, 0, 0xff
+	beq	5f			/* Kuap fault */
+#endif
 4:
 	tophys(r11, r11)
 	rlwimi	r11, r10, 12, 20, 29	/* Create L1 (pgdir/pmd) address */

arch/powerpc/mm/nohash/kup.c

@@ -19,6 +19,8 @@ EXPORT_SYMBOL(disable_kuap_key);
 void setup_kuap(bool disabled)
 {
 	if (disabled) {
+		if (IS_ENABLED(CONFIG_40x))
+			disable_kuep = true;
 		if (smp_processor_id() == boot_cpuid)
 			static_branch_enable(&disable_kuap_key);
 		return;

arch/powerpc/platforms/Kconfig.cputype

@@ -54,6 +54,9 @@ config 40x
 	select PPC_UDBG_16550
 	select 4xx_SOC
 	select HAVE_PCI
+	select PPC_HAVE_KUAP
+	select PPC_HAVE_KUEP
+	select PPC_KUEP if PPC_KUAP
 
 config 44x
 	bool "AMCC 44x, 46x or 47x"
@@ -425,9 +428,9 @@ config PPC_HAVE_KUEP
 	bool
 
 config PPC_KUEP
-	bool "Kernel Userspace Execution Prevention"
+	bool "Kernel Userspace Execution Prevention" if !40x
 	depends on PPC_HAVE_KUEP
-	default y
+	default y if !40x
 	help
 	  Enable support for Kernel Userspace Execution Prevention (KUEP)