powerpc: Add skeleton for Kernel Userspace Execution Prevention

This patch adds a skeleton for Kernel Userspace Execution Prevention.

Then subarches implementing it have to define CONFIG_PPC_HAVE_KUEP
and provide setup_kuep() function.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Don't split strings, use pr_crit_ratelimited()]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/admin-guide/kernel-parameters.txt

@@ -2843,7 +2843,7 @@
 			Disable SMAP (Supervisor Mode Access Prevention)
 			even if it is supported by processor.
 
-	nosmep		[X86]
+	nosmep		[X86,PPC]
 			Disable SMEP (Supervisor Mode Execution Prevention)
 			even if it is supported by processor.
 

arch/powerpc/include/asm/kup.h

@@ -6,6 +6,12 @@
 
 void setup_kup(void);
 
+#ifdef CONFIG_PPC_KUEP
+void setup_kuep(bool disabled);
+#else
+static inline void setup_kuep(bool disabled) { }
+#endif /* CONFIG_PPC_KUEP */
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* _ASM_POWERPC_KUP_H_ */

arch/powerpc/mm/fault.c

@@ -229,11 +229,10 @@ static bool bad_kernel_fault(bool is_exec, unsigned long error_code,
 	/* NX faults set DSISR_PROTFAULT on the 8xx, DSISR_NOEXEC_OR_G on others */
 	if (is_exec && (error_code & (DSISR_NOEXEC_OR_G | DSISR_KEYFAULT |
 				      DSISR_PROTFAULT))) {
-		printk_ratelimited(KERN_CRIT "kernel tried to execute"
-				   " exec-protected page (%lx) -"
-				   "exploit attempt? (uid: %d)\n",
-				   address, from_kuid(&init_user_ns,
-						      current_uid()));
+		pr_crit_ratelimited("kernel tried to execute %s page (%lx) - exploit attempt? (uid: %d)\n",
+				    address >= TASK_SIZE ? "exec-protected" : "user",
+				    address,
+				    from_kuid(&init_user_ns, current_uid()));
 	}
 	return is_exec || (address >= TASK_SIZE);
 }

arch/powerpc/mm/init-common.c

@@ -26,8 +26,19 @@
 #include <asm/pgtable.h>
 #include <asm/kup.h>
 
+static bool disable_kuep = !IS_ENABLED(CONFIG_PPC_KUEP);
+
+static int __init parse_nosmep(char *p)
+{
+	disable_kuep = true;
+	pr_warn("Disabling Kernel Userspace Execution Prevention\n");
+	return 0;
+}
+early_param("nosmep", parse_nosmep);
+
 void __init setup_kup(void)
 {
+	setup_kuep(disable_kuep);
 }
 
 #define CTOR(shift) static void ctor_##shift(void *addr) \

arch/powerpc/platforms/Kconfig.cputype

@@ -345,6 +345,18 @@ config PPC_RADIX_MMU_DEFAULT
 
 	  If you're unsure, say Y.
 
+config PPC_HAVE_KUEP
+	bool
+
+config PPC_KUEP
+	bool "Kernel Userspace Execution Prevention"
+	depends on PPC_HAVE_KUEP
+	default y
+	help
+	  Enable support for Kernel Userspace Execution Prevention (KUEP)
+
+	  If you're unsure, say Y.
+
 config ARCH_ENABLE_HUGEPAGE_MIGRATION
 	def_bool y
 	depends on PPC_BOOK3S_64 && HUGETLB_PAGE && MIGRATION