Date:   Fri, 18 Feb 2022 17:49:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com
Subject: [PATCH 29/29] x86/alternative: Use .ibt_endbr_sites to seal indirect calls

Objtool's --ibt-seal option generates .ibt_endbr_sites which lists
superfluous ENDBR instructions. That is those instructions for which
the function is never indirectly called.

Additionally, objtool's --ibt-fix-direct ensures direct calls never
target an ENDBR instruction.

Combined this yields that these instructions should never be executed.

Poison them using a 4 byte UD1 instruction; for IBT hardware this will
raise an #CP exception due to WAIT-FOR-ENDBR not getting what it
wants. For !IBT hardware it'll trigger #UD.

In either case, it will be 'impossible' to indirectly call these
functions thereafter.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 Makefile                           |    5 ++++
 arch/um/kernel/um_arch.c           |    4 +++
 arch/x86/Kconfig                   |   12 +++++++++
 arch/x86/include/asm/alternative.h |    1 
 arch/x86/include/asm/ibt.h         |    4 ++-
 arch/x86/kernel/alternative.c      |   46 +++++++++++++++++++++++++++++++++++++
 arch/x86/kernel/module.c           |   10 ++++++--
 arch/x86/kernel/traps.c            |   35 ++++++++++++++++++++++++++--
 scripts/Makefile.build             |    3 +-
 scripts/link-vmlinux.sh            |   10 ++++++--
 10 files changed, 122 insertions(+), 8 deletions(-)

--- a/Makefile
+++ b/Makefile
@@ -911,6 +911,11 @@ BUILD_LTO	:= y
 export BUILD_LTO
 endif
 
+ifdef CONFIG_X86_IBT_SEAL
+BUILD_LTO	:= y
+export BUILD_LTO
+endif
+
 ifdef CONFIG_CFI_CLANG
 CC_FLAGS_CFI	:= -fsanitize=cfi \
 		   -fsanitize-cfi-cross-dso \
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -424,6 +424,10 @@ void __init check_bugs(void)
 	os_check_bugs();
 }
 
+void apply_ibt_endbr(s32 *start, s32 *end)
+{
+}
+
 void apply_retpolines(s32 *start, s32 *end)
 {
 }
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1876,6 +1876,18 @@ config X86_IBT
 	  an ENDBR instruction, as such, the compiler will litter the
 	  code with them to make this happen.
 
+config X86_IBT_SEAL
+	prompt "Seal functions"
+	def_bool y
+	depends on X86_IBT && STACK_VALIDATION
+	help
+	  In addition to building the kernel with IBT, seal all functions that
+	  are not indirect call targets, avoiding them ever becomming one.
+
+	  This requires LTO like objtool runs and will slow down the build. It
+	  does significantly reduce the number of ENDBR instructions in the
+	  kernel image as well as provide some validation for !IBT hardware.
+
 config X86_INTEL_MEMORY_PROTECTION_KEYS
 	prompt "Memory Protection Keys"
 	def_bool y
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -76,6 +76,7 @@ extern int alternatives_patched;
 extern void alternative_instructions(void);
 extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
 extern void apply_retpolines(s32 *start, s32 *end);
+extern void apply_ibt_endbr(s32 *start, s32 *end);
 
 struct module;
 
--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -23,8 +23,10 @@
 static inline bool is_endbr(const void *addr)
 {
 	unsigned int val = ~*(unsigned int *)addr;
+	if (val == ~0x0040b90f) /* ud1_endbr */
+		return true;
 	val |= 0x01000000U;
-	return val == ~0xfa1e0ff3;
+	return val == ~0xfa1e0ff3; /* endbr */
 }
 
 extern u64 ibt_save(void);
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -115,6 +115,7 @@ static void __init_or_module add_nops(vo
 }
 
 extern s32 __retpoline_sites[], __retpoline_sites_end[];
+extern s32 __ibt_endbr_sites[], __ibt_endbr_sites_end[];
 extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
 extern s32 __smp_locks[], __smp_locks_end[];
 void text_poke_early(void *addr, const void *opcode, size_t len);
@@ -512,6 +513,49 @@ void __init_or_module noinline apply_ret
 
 #endif /* CONFIG_RETPOLINE && CONFIG_STACK_VALIDATION */
 
+#ifdef CONFIG_X86_IBT_SEAL
+
+/*
+ * ud1    0x0(%rax),%eax -- a 4 byte #UD instruction for when we don't have
+ *                          IBT and still want to trigger fail.
+ */
+static const u8 ud1_endbr[4] = { 0x0f, 0xb9, 0x40, 0x00 };
+
+/*
+ * Generated by: objtool --ibt-seal
+ */
+void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end)
+{
+	s32 *s;
+
+	for (s = start; s < end; s++) {
+		void *addr = (void *)s + *s;
+		u32 endbr;
+
+		if (WARN_ON_ONCE(get_kernel_nofault(endbr, addr)))
+			continue;
+
+		if (WARN_ON_ONCE(!is_endbr(&endbr)))
+			continue;
+
+		DPRINTK("ENDBR at: %pS (%px)", addr, addr);
+
+		/*
+		 * When we have IBT, the lack of ENDBR will trigger #CP
+		 * When we don't have IBT, explicitly trigger #UD
+		 */
+		DUMP_BYTES(((u8*)addr), 4, "%px: orig: ", addr);
+		DUMP_BYTES(((u8*)ud1_endbr), 4, "%px: repl: ", addr);
+		text_poke_early(addr, ud1_endbr, 4);
+	}
+}
+
+#else
+
+void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end) { }
+
+#endif /* CONFIG_X86_IBT_SEAL */
+
 #ifdef CONFIG_SMP
 static void alternatives_smp_lock(const s32 *start, const s32 *end,
 				  u8 *text, u8 *text_end)
@@ -832,6 +876,8 @@ void __init alternative_instructions(voi
 	 */
 	apply_alternatives(__alt_instructions, __alt_instructions_end);
 
+	apply_ibt_endbr(__ibt_endbr_sites, __ibt_endbr_sites_end);
+
 #ifdef CONFIG_SMP
 	/* Patch to UP if other cpus not imminent. */
 	if (!noreplace_smp && (num_present_cpus() == 1 || setup_max_cpus <= 1)) {
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -132,7 +132,7 @@ int apply_relocate(Elf32_Shdr *sechdrs,
 
 static inline void ibt_fix_direct(void *loc, u64 *val)
 {
-#ifdef CONFIG_X86_IBT
+#ifdef CONFIG_X86_IBT_SEAL
 	const void *addr = (void *)(4 + *val);
 	union text_poke_insn text;
 	u32 insn;
@@ -287,7 +287,7 @@ int module_finalize(const Elf_Ehdr *hdr,
 {
 	const Elf_Shdr *s, *text = NULL, *alt = NULL, *locks = NULL,
 		*para = NULL, *orc = NULL, *orc_ip = NULL,
-		*retpolines = NULL;
+		*retpolines = NULL, *ibt_endbr = NULL;
 	char *secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
 
 	for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
@@ -305,6 +305,8 @@ int module_finalize(const Elf_Ehdr *hdr,
 			orc_ip = s;
 		if (!strcmp(".retpoline_sites", secstrings + s->sh_name))
 			retpolines = s;
+		if (!strcmp(".ibt_endbr_sites", secstrings + s->sh_name))
+			ibt_endbr = s;
 	}
 
 	if (para) {
@@ -320,6 +322,10 @@ int module_finalize(const Elf_Ehdr *hdr,
 		void *aseg = (void *)alt->sh_addr;
 		apply_alternatives(aseg, aseg + alt->sh_size);
 	}
+	if (ibt_endbr) {
+		void *iseg = (void *)ibt_endbr->sh_addr;
+		apply_ibt_endbr(iseg, iseg + ibt_endbr->sh_size);
+	}
 	if (locks && text) {
 		void *lseg = (void *)locks->sh_addr;
 		void *tseg = (void *)text->sh_addr;
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -214,6 +214,12 @@ DEFINE_IDTENTRY(exc_overflow)
 
 static bool ibt_fatal = true;
 
+static void handle_endbr(struct pt_regs *regs)
+{
+	pr_err("Missing ENDBR: %pS\n", (void *)instruction_pointer(regs));
+	BUG_ON(ibt_fatal);
+}
+
 extern unsigned long ibt_selftest_ip; /* defined in asm beow */
 static volatile bool ibt_selftest_ok = false;
 
@@ -232,8 +238,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_control_pr
 		return;
 	}
 
-	pr_err("Missing ENDBR: %pS\n", (void *)instruction_pointer(regs));
-	BUG_ON(ibt_fatal);
+	handle_endbr(regs);
 }
 
 bool ibt_selftest(void)
@@ -277,6 +282,29 @@ static int __init ibt_setup(char *str)
 
 __setup("ibt=", ibt_setup);
 
+static bool handle_ud1_endbr(struct pt_regs *regs)
+{
+	u32 ud1;
+
+	if (get_kernel_nofault(ud1, (u32 *)regs->ip))
+		return false;
+
+	if (ud1 == 0x0040b90f) {
+		handle_endbr(regs);
+		regs->ip += 4;
+		return true;
+	}
+
+	return false;
+}
+
+#else /* CONFIG_X86_IBT */
+
+static bool handle_ud1_endbr(struct pt_regs *regs)
+{
+	return false;
+}
+
 #endif /* CONFIG_X86_IBT */
 
 #ifdef CONFIG_X86_F00F_BUG
@@ -285,6 +313,9 @@ void handle_invalid_op(struct pt_regs *r
 static inline void handle_invalid_op(struct pt_regs *regs)
 #endif
 {
+	if (!user_mode(regs) && handle_ud1_endbr(regs))
+		return;
+
 	do_error_trap(regs, 0, "invalid opcode", X86_TRAP_UD, SIGILL,
 		      ILL_ILLOPN, error_get_trap_addr(regs));
 }
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -231,6 +231,7 @@ objtool_args =								\
 	$(if $(CONFIG_UNWINDER_ORC),orc generate,check)			\
 	$(if $(part-of-module), --module)				\
 	$(if $(BUILD_LTO), --lto)					\
+	$(if $(CONFIG_X86_IBT_SEAL), --ibt --ibt-fix-direct --ibt-seal)	\
 	$(if $(CONFIG_FRAME_POINTER),, --no-fp)				\
 	$(if $(CONFIG_GCOV_KERNEL)$(CONFIG_LTO_CLANG), --no-unreachable)\
 	$(if $(CONFIG_RETPOLINE), --retpoline)				\
@@ -305,7 +306,7 @@ quiet_cmd_cc_lto_link_modules = LTO [M]
 		--whole-archive $(filter-out FORCE,$^)			\
 		$(cmd_objtool)
 else
-quiet_cmd_cc_lto_link_modules = LD [M] $@
+quiet_cmd_cc_lto_link_modules = LD [M]  $@
       cmd_cc_lto_link_modules =						\
 	$(LD) $(ld_flags) -r -o $@					\
 		$(filter-out FORCE,$^)					\
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -108,7 +108,9 @@ objtool_link()
 	local objtoolcmd;
 	local objtoolopt;
 
-	if is_enabled CONFIG_LTO_CLANG && is_enabled CONFIG_STACK_VALIDATION; then
+	if is_enabled CONFIG_STACK_VALIDATION && \
+	   ( is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_IBT_SEAL ); then
+
 		# Don't perform vmlinux validation unless explicitly requested,
 		# but run objtool on vmlinux.o now that we have an object file.
 		if is_enabled CONFIG_UNWINDER_ORC; then
@@ -117,6 +119,10 @@ objtool_link()
 
 		objtoolopt="${objtoolopt} --lto"
 
+		if is_enabled CONFIG_X86_IBT_SEAL; then
+			objtoolopt="${objtoolopt} --ibt --ibt-fix-direct --ibt-seal"
+		fi
+
 		if is_enabled CONFIG_FTRACE_MCOUNT_USE_OBJTOOL; then
 			objtoolopt="${objtoolopt} --mcount"
 		fi
@@ -168,7 +174,7 @@ vmlinux_link()
 	# skip output file argument
 	shift
 
-	if is_enabled CONFIG_LTO_CLANG; then
+	if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_IBT_SEAL; then
 		# Use vmlinux.o instead of performing the slow LTO link again.
 		objs=vmlinux.o
 		libs=

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