s390: add automatic detection of the spectre defense

Automatically decide between nobp vs. expolines if the spectre_v2=auto
kernel parameter is specified or CONFIG_EXPOLINE_AUTO=y is set.

The decision made at boot time due to CONFIG_EXPOLINE_AUTO=y being set
can be overruled with the nobp, nospec and spectre_v2 kernel parameters.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -576,7 +576,7 @@ choice
 config EXPOLINE_OFF
 	bool "spectre_v2=off"
 
-config EXPOLINE_MEDIUM
+config EXPOLINE_AUTO
 	bool "spectre_v2=auto"
 
 config EXPOLINE_FULL

arch/s390/Makefile

@@ -81,7 +81,7 @@ ifdef CONFIG_EXPOLINE
     CC_FLAGS_EXPOLINE += -mfunction-return=thunk
     CC_FLAGS_EXPOLINE += -mindirect-branch-table
     export CC_FLAGS_EXPOLINE
-    cflags-y += $(CC_FLAGS_EXPOLINE)
+    cflags-y += $(CC_FLAGS_EXPOLINE) -DCC_USING_EXPOLINE
   endif
 endif
 

arch/s390/include/asm/nospec-branch.h

@@ -6,12 +6,10 @@
 
 #include <linux/types.h>
 
-extern int nospec_call_disable;
-extern int nospec_return_disable;
+extern int nospec_disable;
 
 void nospec_init_branches(void);
-void nospec_call_revert(s32 *start, s32 *end);
-void nospec_return_revert(s32 *start, s32 *end);
+void nospec_revert(s32 *start, s32 *end);
 
 #endif /* __ASSEMBLY__ */
 

arch/s390/kernel/alternative.c

@@ -2,6 +2,7 @@
 #include <linux/module.h>
 #include <asm/alternative.h>
 #include <asm/facility.h>
+#include <asm/nospec-branch.h>
 
 #define MAX_PATCH_LEN (255 - 1)
 

arch/s390/kernel/module.c

@@ -159,7 +159,7 @@ int module_frob_arch_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
 	me->core_layout.size += me->arch.got_size;
 	me->arch.plt_offset = me->core_layout.size;
 	if (me->arch.plt_size) {
-		if (IS_ENABLED(CONFIG_EXPOLINE) && !nospec_call_disable)
+		if (IS_ENABLED(CONFIG_EXPOLINE) && !nospec_disable)
 			me->arch.plt_size += PLT_ENTRY_SIZE;
 		me->core_layout.size += me->arch.plt_size;
 	}
@@ -318,8 +318,7 @@ static int apply_rela(Elf_Rela *rela, Elf_Addr base, Elf_Sym *symtab,
 				info->plt_offset;
 			ip[0] = 0x0d10e310;	/* basr 1,0  */
 			ip[1] = 0x100a0004;	/* lg	1,10(1) */
-			if (IS_ENABLED(CONFIG_EXPOLINE) &&
-			    !nospec_call_disable) {
+			if (IS_ENABLED(CONFIG_EXPOLINE) && !nospec_disable) {
 				unsigned int *ij;
 				ij = me->core_layout.base +
 					me->arch.plt_offset +
@@ -440,7 +439,7 @@ int module_finalize(const Elf_Ehdr *hdr,
 	void *aseg;
 
 	if (IS_ENABLED(CONFIG_EXPOLINE) &&
-	    !nospec_call_disable && me->arch.plt_size) {
+	    !nospec_disable && me->arch.plt_size) {
 		unsigned int *ij;
 
 		ij = me->core_layout.base + me->arch.plt_offset +
@@ -467,11 +466,11 @@ int module_finalize(const Elf_Ehdr *hdr,
 
 		if (IS_ENABLED(CONFIG_EXPOLINE) &&
 		    (!strcmp(".nospec_call_table", secname)))
-			nospec_call_revert(aseg, aseg + s->sh_size);
+			nospec_revert(aseg, aseg + s->sh_size);
 
 		if (IS_ENABLED(CONFIG_EXPOLINE) &&
 		    (!strcmp(".nospec_return_table", secname)))
-			nospec_return_revert(aseg, aseg + s->sh_size);
+			nospec_revert(aseg, aseg + s->sh_size);
 	}
 
 	jump_label_apply_nops(me);

arch/s390/kernel/nospec-branch.c

@@ -10,10 +10,17 @@ static int __init nobp_setup_early(char *str)
 	rc = kstrtobool(str, &enabled);
 	if (rc)
 		return rc;
-	if (enabled && test_facility(82))
+	if (enabled && test_facility(82)) {
+		/*
+		 * The user explicitely requested nobp=1, enable it and
+		 * disable the expoline support.
+		 */
 		__set_facility(82, S390_lowcore.alt_stfle_fac_list);
-	else
+		if (IS_ENABLED(CONFIG_EXPOLINE))
+			nospec_disable = 1;
+	} else {
 		__clear_facility(82, S390_lowcore.alt_stfle_fac_list);
+	}
 	return 0;
 }
 early_param("nobp", nobp_setup_early);
@@ -27,31 +34,46 @@ early_param("nospec", nospec_setup_early);
 
 #ifdef CONFIG_EXPOLINE
 
-int nospec_call_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);
-int nospec_return_disable = !IS_ENABLED(CONFIG_EXPOLINE_FULL);
+int nospec_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);
 
 static int __init nospectre_v2_setup_early(char *str)
 {
-	nospec_call_disable = 1;
-	nospec_return_disable = 1;
+	nospec_disable = 1;
 	return 0;
 }
 early_param("nospectre_v2", nospectre_v2_setup_early);
 
+static int __init spectre_v2_auto_early(void)
+{
+	if (IS_ENABLED(CC_USING_EXPOLINE)) {
+		/*
+		 * The kernel has been compiled with expolines.
+		 * Keep expolines enabled and disable nobp.
+		 */
+		nospec_disable = 0;
+		__clear_facility(82, S390_lowcore.alt_stfle_fac_list);
+	}
+	/*
+	 * If the kernel has not been compiled with expolines the
+	 * nobp setting decides what is done, this depends on the
+	 * CONFIG_KERNEL_NP option and the nobp/nospec parameters.
+	 */
+	return 0;
+}
+#ifdef CONFIG_EXPOLINE_AUTO
+early_initcall(spectre_v2_auto_early);
+#endif
+
 static int __init spectre_v2_setup_early(char *str)
 {
 	if (str && !strncmp(str, "on", 2)) {
-		nospec_call_disable = 0;
-		nospec_return_disable = 0;
-	}
-	if (str && !strncmp(str, "off", 3)) {
-		nospec_call_disable = 1;
-		nospec_return_disable = 1;
-	}
-	if (str && !strncmp(str, "auto", 4)) {
-		nospec_call_disable = 0;
-		nospec_return_disable = 1;
+		nospec_disable = 0;
+		__clear_facility(82, S390_lowcore.alt_stfle_fac_list);
 	}
+	if (str && !strncmp(str, "off", 3))
+		nospec_disable = 1;
+	if (str && !strncmp(str, "auto", 4))
+		spectre_v2_auto_early();
 	return 0;
 }
 early_param("spectre_v2", spectre_v2_setup_early);
@@ -104,15 +126,9 @@ static void __init_or_module __nospec_revert(s32 *start, s32 *end)
 	}
 }
 
-void __init_or_module nospec_call_revert(s32 *start, s32 *end)
-{
-	if (nospec_call_disable)
-		__nospec_revert(start, end);
-}
-
-void __init_or_module nospec_return_revert(s32 *start, s32 *end)
+void __init_or_module nospec_revert(s32 *start, s32 *end)
 {
-	if (nospec_return_disable)
+	if (nospec_disable)
 		__nospec_revert(start, end);
 }
 
@@ -120,8 +136,8 @@ extern s32 __nospec_call_start[], __nospec_call_end[];
 extern s32 __nospec_return_start[], __nospec_return_end[];
 void __init nospec_init_branches(void)
 {
-	nospec_call_revert(__nospec_call_start, __nospec_call_end);
-	nospec_return_revert(__nospec_return_start, __nospec_return_end);
+	nospec_revert(__nospec_call_start, __nospec_call_end);
+	nospec_revert(__nospec_return_start, __nospec_return_end);
 }
 
 #endif /* CONFIG_EXPOLINE */