x86/retbleed: Add fine grained Kconfig knobs

Do fine-grained Kconfig for all the various retbleed parts.

NOTE: if your compiler doesn't support return thunks this will
silently 'upgrade' your mitigation to IBPB, you might not like this.

Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>

arch/x86/Kconfig

···
 	def_bool y
 	depends on X86_GOLDFISH
 
-config RETPOLINE
-	bool "Avoid speculative indirect branches in kernel"
-	select OBJTOOL if HAVE_OBJTOOL
-	default y
-	help
-	  Compile kernel with the retpoline compiler options to guard against
-	  kernel-to-user data leaks by avoiding speculative indirect
-	  branches. Requires a compiler with -mindirect-branch=thunk-extern
-	  support for full protection. The kernel may run slower.
-
-config CC_HAS_SLS
-	def_bool $(cc-option,-mharden-sls=all)
-
-config CC_HAS_RETURN_THUNK
-	def_bool $(cc-option,-mfunction-return=thunk-extern)
-
-config SLS
-	bool "Mitigate Straight-Line-Speculation"
-	depends on CC_HAS_SLS && X86_64
-	select OBJTOOL if HAVE_OBJTOOL
-	default n
-	help
-	  Compile the kernel with straight-line-speculation options to guard
-	  against straight line speculation. The kernel image might be slightly
-	  larger.
-
 config X86_CPU_RESCTRL
 	bool "x86 CPU resource control support"
 	depends on X86 && (CPU_SUP_INTEL || CPU_SUP_AMD)
···
 source "kernel/livepatch/Kconfig"
 
 endmenu
+
+config CC_HAS_SLS
+	def_bool $(cc-option,-mharden-sls=all)
+
+config CC_HAS_RETURN_THUNK
+	def_bool $(cc-option,-mfunction-return=thunk-extern)
+
+menuconfig SPECULATION_MITIGATIONS
+	bool "Mitigations for speculative execution vulnerabilities"
+	default y
+	help
+	  Say Y here to enable options which enable mitigations for
+	  speculative execution hardware vulnerabilities.
+
+	  If you say N, all mitigations will be disabled. You really
+	  should know what you are doing to say so.
+
+if SPECULATION_MITIGATIONS
+
+config PAGE_TABLE_ISOLATION
+	bool "Remove the kernel mapping in user mode"
+	default y
+	depends on (X86_64 || X86_PAE)
+	help
+	  This feature reduces the number of hardware side channels by
+	  ensuring that the majority of kernel addresses are not mapped
+	  into userspace.
+
+	  See Documentation/x86/pti.rst for more details.
+
+config RETPOLINE
+	bool "Avoid speculative indirect branches in kernel"
+	select OBJTOOL if HAVE_OBJTOOL
+	default y
+	help
+	  Compile kernel with the retpoline compiler options to guard against
+	  kernel-to-user data leaks by avoiding speculative indirect
+	  branches. Requires a compiler with -mindirect-branch=thunk-extern
+	  support for full protection. The kernel may run slower.
+
+config RETHUNK
+	bool "Enable return-thunks"
+	depends on RETPOLINE && CC_HAS_RETURN_THUNK
+	select OBJTOOL if HAVE_OBJTOOL
+	default y
+	help
+	  Compile the kernel with the return-thunks compiler option to guard
+	  against kernel-to-user data leaks by avoiding return speculation.
+	  Requires a compiler with -mfunction-return=thunk-extern
+	  support for full protection. The kernel may run slower.
+
+config CPU_UNRET_ENTRY
+	bool "Enable UNRET on kernel entry"
+	depends on CPU_SUP_AMD && RETHUNK
+	default y
+	help
+	  Compile the kernel with support for the retbleed=unret mitigation.
+
+config CPU_IBPB_ENTRY
+	bool "Enable IBPB on kernel entry"
+	depends on CPU_SUP_AMD
+	default y
+	help
+	  Compile the kernel with support for the retbleed=ibpb mitigation.
+
+config CPU_IBRS_ENTRY
+	bool "Enable IBRS on kernel entry"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Compile the kernel with support for the spectre_v2=ibrs mitigation.
+	  This mitigates both spectre_v2 and retbleed at great cost to
+	  performance.
+
+config SLS
+	bool "Mitigate Straight-Line-Speculation"
+	depends on CC_HAS_SLS && X86_64
+	select OBJTOOL if HAVE_OBJTOOL
+	default n
+	help
+	  Compile the kernel with straight-line-speculation options to guard
+	  against straight line speculation. The kernel image might be slightly
+	  larger.
+
+endif
 
 config ARCH_HAS_ADD_PAGES
 	def_bool y

arch/x86/Makefile

···
 ifdef CONFIG_CC_IS_GCC
 RETPOLINE_CFLAGS	:= $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
 RETPOLINE_CFLAGS	+= $(call cc-option,-mindirect-branch-cs-prefix)
-RETPOLINE_CFLAGS	+= $(call cc-option,-mfunction-return=thunk-extern)
 RETPOLINE_VDSO_CFLAGS	:= $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
 endif
 ifdef CONFIG_CC_IS_CLANG
 RETPOLINE_CFLAGS	:= -mretpoline-external-thunk
 RETPOLINE_VDSO_CFLAGS	:= -mretpoline
-RETPOLINE_CFLAGS	+= $(call cc-option,-mfunction-return=thunk-extern)
 endif
+
+ifdef CONFIG_RETHUNK
+RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern
+RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)
+endif
+
 export RETPOLINE_CFLAGS
 export RETPOLINE_VDSO_CFLAGS
 

arch/x86/entry/calling.h

···
  * Assumes x86_spec_ctrl_{base,current} to have SPEC_CTRL_IBRS set.
  */
 .macro IBRS_ENTER save_reg
+#ifdef CONFIG_CPU_IBRS_ENTRY
 	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_KERNEL_IBRS
 	movl	$MSR_IA32_SPEC_CTRL, %ecx
 
···
 	shr	$32, %rdx
 	wrmsr
 .Lend_\@:
+#endif
 .endm
 
 /*
···
  * regs. Must be called after the last RET.
  */
 .macro IBRS_EXIT save_reg
+#ifdef CONFIG_CPU_IBRS_ENTRY
 	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_KERNEL_IBRS
 	movl	$MSR_IA32_SPEC_CTRL, %ecx
 
···
 	shr	$32, %rdx
 	wrmsr
 .Lend_\@:
+#endif
 .endm
 
 /*

arch/x86/include/asm/disabled-features.h

···
 # define DISABLE_RETPOLINE	0
 #else
 # define DISABLE_RETPOLINE	((1 << (X86_FEATURE_RETPOLINE & 31)) | \
-				 (1 << (X86_FEATURE_RETPOLINE_LFENCE & 31)) | \
-				 (1 << (X86_FEATURE_RETHUNK & 31)) | \
-				 (1 << (X86_FEATURE_UNRET & 31)))
+				 (1 << (X86_FEATURE_RETPOLINE_LFENCE & 31)))
+#endif
+
+#ifdef CONFIG_RETHUNK
+# define DISABLE_RETHUNK	0
+#else
+# define DISABLE_RETHUNK	(1 << (X86_FEATURE_RETHUNK & 31))
+#endif
+
+#ifdef CONFIG_CPU_UNRET_ENTRY
+# define DISABLE_UNRET		0
+#else
+# define DISABLE_UNRET		(1 << (X86_FEATURE_UNRET & 31))
 #endif
 
 #ifdef CONFIG_INTEL_IOMMU_SVM
···
 #define DISABLED_MASK8	(DISABLE_TDX_GUEST)
 #define DISABLED_MASK9	(DISABLE_SGX)
 #define DISABLED_MASK10	0
-#define DISABLED_MASK11	(DISABLE_RETPOLINE)
+#define DISABLED_MASK11	(DISABLE_RETPOLINE|DISABLE_RETHUNK|DISABLE_UNRET)
 #define DISABLED_MASK12	0
 #define DISABLED_MASK13	0
 #define DISABLED_MASK14	0

arch/x86/include/asm/linkage.h

···
 #define __ALIGN_STR	__stringify(__ALIGN)
 #endif
 
-#if defined(CONFIG_RETPOLINE) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
+#if defined(CONFIG_RETHUNK) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
 #define RET	jmp __x86_return_thunk
 #else /* CONFIG_RETPOLINE */
 #ifdef CONFIG_SLS
···
 
 #else /* __ASSEMBLY__ */
 
-#if defined(CONFIG_RETPOLINE) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
+#if defined(CONFIG_RETHUNK) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
 #define ASM_RET	"jmp __x86_return_thunk\n\t"
 #else /* CONFIG_RETPOLINE */
 #ifdef CONFIG_SLS

arch/x86/include/asm/nospec-branch.h

···
 .Lskip_rsb_\@:
 .endm
 
+#ifdef CONFIG_CPU_UNRET_ENTRY
+#define CALL_ZEN_UNTRAIN_RET	"call zen_untrain_ret"
+#else
+#define CALL_ZEN_UNTRAIN_RET	""
+#endif
+
 /*
  * Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
  * return thunk isn't mapped into the userspace tables (then again, AMD
···
  * where we have a stack but before any RET instruction.
  */
 .macro UNTRAIN_RET
-#ifdef CONFIG_RETPOLINE
+#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY)
 	ANNOTATE_UNRET_END
 	ALTERNATIVE_2 "",						\
-	              "call zen_untrain_ret", X86_FEATURE_UNRET,	\
+	              CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET,		\
 		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB
 #endif
 .endm

arch/x86/include/asm/static_call.h

···
 #define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)			\
 	__ARCH_DEFINE_STATIC_CALL_TRAMP(name, ".byte 0xe9; .long " #func " - (. + 4)")
 
-#ifdef CONFIG_RETPOLINE
+#ifdef CONFIG_RETHUNK
 #define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)			\
 	__ARCH_DEFINE_STATIC_CALL_TRAMP(name, "jmp __x86_return_thunk")
 #else

arch/x86/kernel/alternative.c

···
 	}
 }
 
+#ifdef CONFIG_RETHUNK
 /*
  * Rewrite the compiler generated return thunk tail-calls.
  *
···
 		}
 	}
 }
+#else
+void __init_or_module noinline apply_returns(s32 *start, s32 *end) { }
+#endif /* CONFIG_RETHUNK */
+
 #else /* !CONFIG_RETPOLINE || !CONFIG_OBJTOOL */
 
 void __init_or_module noinline apply_retpolines(s32 *start, s32 *end) { }

arch/x86/kernel/cpu/amd.c

···
 
 void init_spectral_chicken(struct cpuinfo_x86 *c)
 {
+#ifdef CONFIG_CPU_UNRET_ENTRY
 	u64 value;
 
 	/*
···
 			wrmsrl_safe(MSR_ZEN2_SPECTRAL_CHICKEN, value);
 		}
 	}
+#endif
 }
 
 static void init_amd_zn(struct cpuinfo_x86 *c)

arch/x86/kernel/cpu/bugs.c

···
 early_param("retbleed", retbleed_parse_cmdline);
 
 #define RETBLEED_UNTRAIN_MSG "WARNING: BTB untrained return thunk mitigation is only effective on AMD/Hygon!\n"
-#define RETBLEED_COMPILER_MSG "WARNING: kernel not compiled with RETPOLINE or -mfunction-return capable compiler; falling back to IBPB!\n"
 #define RETBLEED_INTEL_MSG "WARNING: Spectre v2 mitigation leaves CPU vulnerable to RETBleed attacks, data leaks possible!\n"
 
 static void __init retbleed_select_mitigation(void)
···
 		return;
 
 	case RETBLEED_CMD_UNRET:
-		retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+		if (IS_ENABLED(CONFIG_CPU_UNRET_ENTRY)) {
+			retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+		} else {
+			pr_err("WARNING: kernel not compiled with CPU_UNRET_ENTRY.\n");
+			goto do_cmd_auto;
+		}
 		break;
 
 	case RETBLEED_CMD_IBPB:
-		retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+		if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
+			retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+		} else {
+			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
+			goto do_cmd_auto;
+		}
 		break;
 
+do_cmd_auto:
 	case RETBLEED_CMD_AUTO:
 	default:
 		if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
-		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
-			retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
+			if (IS_ENABLED(CONFIG_CPU_UNRET_ENTRY))
+				retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+			else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY))
+				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+		}
 
 		/*
 		 * The Intel mitigation (IBRS or eIBRS) was already selected in
···
 
 	switch (retbleed_mitigation) {
 	case RETBLEED_MITIGATION_UNRET:
-
-		if (!IS_ENABLED(CONFIG_RETPOLINE) ||
-		    !IS_ENABLED(CONFIG_CC_HAS_RETURN_THUNK)) {
-			pr_err(RETBLEED_COMPILER_MSG);
-			retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
-			goto retbleed_force_ibpb;
-		}
-
 		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
 		setup_force_cpu_cap(X86_FEATURE_UNRET);
 
···
 		break;
 
 	case RETBLEED_MITIGATION_IBPB:
-retbleed_force_ibpb:
 		setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
 		mitigate_smt = true;
 		break;
···
 		return SPECTRE_V2_CMD_AUTO;
 	}
 
+	if (cmd == SPECTRE_V2_CMD_IBRS && !IS_ENABLED(CONFIG_CPU_IBRS_ENTRY)) {
+		pr_err("%s selected but not compiled in. Switching to AUTO select\n",
+		       mitigation_options[i].option);
+		return SPECTRE_V2_CMD_AUTO;
+	}
+
 	if (cmd == SPECTRE_V2_CMD_IBRS && boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
 		pr_err("%s selected but not Intel CPU. Switching to AUTO select\n",
 		       mitigation_options[i].option);
···
 			break;
 		}
 
-		if (boot_cpu_has_bug(X86_BUG_RETBLEED) &&
+		if (IS_ENABLED(CONFIG_CPU_IBRS_ENTRY) &&
+		    boot_cpu_has_bug(X86_BUG_RETBLEED) &&
 		    retbleed_cmd != RETBLEED_CMD_OFF &&
 		    boot_cpu_has(X86_FEATURE_IBRS) &&
 		    boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {

arch/x86/kernel/static_call.c

···
 }
 EXPORT_SYMBOL_GPL(arch_static_call_transform);
 
-#ifdef CONFIG_RETPOLINE
+#ifdef CONFIG_RETHUNK
 /*
  * This is called by apply_returns() to fix up static call trampolines,
  * specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as

arch/x86/kvm/emulate.c

···
  *
  * ENDBR			[4 bytes; CONFIG_X86_KERNEL_IBT]
  * SETcc %al			[3 bytes]
- * RET | JMP __x86_return_thunk	[1,5 bytes; CONFIG_RETPOLINE]
+ * RET | JMP __x86_return_thunk	[1,5 bytes; CONFIG_RETHUNK]
  * INT3				[1 byte; CONFIG_SLS]
  */
-#define RET_LENGTH	(1 + (4 * IS_ENABLED(CONFIG_RETPOLINE)) + \
+#define RET_LENGTH	(1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \
 			 IS_ENABLED(CONFIG_SLS))
 #define SETCC_LENGTH	(ENDBR_INSN_SIZE + 3 + RET_LENGTH)
 #define SETCC_ALIGN	(4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1))

arch/x86/lib/retpoline.S

···
  * This function name is magical and is used by -mfunction-return=thunk-extern
  * for the compiler to generate JMPs to it.
  */
+#ifdef CONFIG_RETHUNK
+
 	.section .text.__x86.return_thunk
 
 /*
···
 __EXPORT_THUNK(zen_untrain_ret)
 
 EXPORT_SYMBOL(__x86_return_thunk)
+
+#endif /* CONFIG_RETHUNK */

scripts/Makefile.lib

···
 	$(if $(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL), --mcount)		\
 	$(if $(CONFIG_UNWINDER_ORC), --orc)				\
 	$(if $(CONFIG_RETPOLINE), --retpoline)				\
+	$(if $(CONFIG_RETHUNK), --rethunk)				\
 	$(if $(CONFIG_SLS), --sls)					\
 	$(if $(CONFIG_STACK_VALIDATION), --stackval)			\
 	$(if $(CONFIG_HAVE_STATIC_CALL_INLINE), --static-call)		\

scripts/Makefile.vmlinux_o

···
 
 objtool_args := \
 	$(if $(delay-objtool),$(objtool_args)) \
-	$(if $(CONFIG_NOINSTR_VALIDATION), --noinstr $(if $(CONFIG_RETPOLINE), --unret)) \
+	$(if $(CONFIG_NOINSTR_VALIDATION), --noinstr $(if $(CONFIG_CPU_UNRET_ENTRY), --unret)) \
 	$(if $(CONFIG_GCOV_KERNEL), --no-unreachable) \
 	--link
 

security/Kconfig

···
 	  implement socket and networking access controls.
 	  If you are unsure how to answer this question, answer N.
 
-config PAGE_TABLE_ISOLATION
-	bool "Remove the kernel mapping in user mode"
-	default y
-	depends on (X86_64 || X86_PAE) && !UML
-	help
-	  This feature reduces the number of hardware side channels by
-	  ensuring that the majority of kernel addresses are not mapped
-	  into userspace.
-
-	  See Documentation/x86/pti.rst for more details.
-
 config SECURITY_INFINIBAND
 	bool "Infiniband Security Hooks"
 	depends on SECURITY && INFINIBAND

tools/objtool/builtin-check.c

···
 	OPT_BOOLEAN('n', "noinstr", &opts.noinstr, "validate noinstr rules"),
 	OPT_BOOLEAN('o', "orc", &opts.orc, "generate ORC metadata"),
 	OPT_BOOLEAN('r', "retpoline", &opts.retpoline, "validate and annotate retpoline usage"),
+	OPT_BOOLEAN(0,   "rethunk", &opts.rethunk, "validate and annotate rethunk usage"),
 	OPT_BOOLEAN(0,   "unret", &opts.unret, "validate entry unret placement"),
 	OPT_BOOLEAN('l', "sls", &opts.sls, "validate straight-line-speculation mitigations"),
 	OPT_BOOLEAN('s', "stackval", &opts.stackval, "validate frame pointer rules"),
···
 	    opts.noinstr		||
 	    opts.orc			||
 	    opts.retpoline		||
+	    opts.rethunk		||
 	    opts.sls			||
 	    opts.stackval		||
 	    opts.static_call		||
···
 		}
 
 		return true;
+	}
+
+	if (opts.unret && !opts.rethunk) {
+		ERROR("--unret requires --rethunk");
+		return false;
 	}
 
 	if (opts.dump_orc)

tools/objtool/check.c

···
 			continue;
 
 		if (insn->type == INSN_RETURN) {
-			WARN_FUNC("'naked' return found in RETPOLINE build",
-				  insn->sec, insn->offset);
+			if (opts.rethunk) {
+				WARN_FUNC("'naked' return found in RETHUNK build",
+					  insn->sec, insn->offset);
+			} else
+				continue;
 		} else {
 			WARN_FUNC("indirect %s found in RETPOLINE build",
 				  insn->sec, insn->offset,
···
 		if (ret < 0)
 			goto out;
 		warnings += ret;
+	}
 
+	if (opts.rethunk) {
 		ret = create_return_sites_sections(file);
 		if (ret < 0)
 			goto out;

tools/objtool/include/objtool/builtin.h

···
 	bool noinstr;
 	bool orc;
 	bool retpoline;
+	bool rethunk;
 	bool unret;
 	bool sls;
 	bool stackval;