powerpc/book3s64/kup: Check max key supported before enabling kup

Don't enable KUEP/KUAP if we support less than or equal to 3 keys.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201202043854.76406-1-aneesh.kumar@linux.ibm.com

arch/powerpc/include/asm/kup.h

@@ -44,6 +44,9 @@
 
 #else /* !__ASSEMBLY__ */
 
+extern bool disable_kuep;
+extern bool disable_kuap;
+
 #include <linux/pgtable.h>
 
 void setup_kup(void);

arch/powerpc/mm/book3s64/pkeys.c

@@ -185,6 +185,27 @@ void __init pkey_early_init_devtree(void)
 		default_uamor &= ~(0x3ul << pkeyshift(execute_only_key));
 	}
 
+	if (unlikely(num_pkey <= 3)) {
+		/*
+		 * Insufficient number of keys to support
+		 * KUAP/KUEP feature.
+		 */
+		disable_kuep = true;
+		disable_kuap = true;
+		WARN(1, "Disabling kernel user protection due to low (%d) max supported keys\n", num_pkey);
+	} else {
+		/*  handle key which is used by kernel for KAUP */
+		reserved_allocation_mask |= (0x1 << 3);
+		/*
+		 * Mark access for kup_key in default amr so that
+		 * we continue to operate with that AMR in
+		 * copy_to/from_user().
+		 */
+		default_amr   &= ~(0x3ul << pkeyshift(3));
+		default_iamr  &= ~(0x1ul << pkeyshift(3));
+		default_uamor &= ~(0x3ul << pkeyshift(3));
+	}
+
 	/*
 	 * Allow access for only key 0. And prevent any other modification.
 	 */
@@ -205,18 +226,6 @@ void __init pkey_early_init_devtree(void)
 	reserved_allocation_mask |= (0x1 << 1);
 	default_uamor &= ~(0x3ul << pkeyshift(1));
 
-	/*  handle key which is used by kernel for KAUP */
-	reserved_allocation_mask |= (0x1 << 3);
-	/*
-	 * Mark access for KUAP key in default amr so that
-	 * we continue to operate with that AMR in
-	 * copy_to/from_user().
-	 */
-	default_amr   &= ~(0x3ul << pkeyshift(3));
-	default_iamr  &= ~(0x1ul << pkeyshift(3));
-	default_uamor &= ~(0x3ul << pkeyshift(3));
-
-
 	/*
 	 * Prevent the usage of OS reserved keys. Update UAMOR
 	 * for those keys. Also mark the rest of the bits in the

arch/powerpc/mm/init-common.c

@@ -28,8 +28,8 @@ EXPORT_SYMBOL_GPL(kernstart_addr);
 unsigned long kernstart_virt_addr __ro_after_init = KERNELBASE;
 EXPORT_SYMBOL_GPL(kernstart_virt_addr);
 
-static bool disable_kuep = !IS_ENABLED(CONFIG_PPC_KUEP);
-static bool disable_kuap = !IS_ENABLED(CONFIG_PPC_KUAP);
+bool disable_kuep = !IS_ENABLED(CONFIG_PPC_KUEP);
+bool disable_kuap = !IS_ENABLED(CONFIG_PPC_KUAP);
 
 static int __init parse_nosmep(char *p)
 {