objtool: Find unused ENDBR instructions

Find all ENDBR instructions which are never referenced and stick them
in a section such that the kernel can poison them, sealing the
functions from ever being an indirect call target.

This removes about 1-in-4 ENDBR instructions.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.763643193@infradead.org

arch/x86/kernel/vmlinux.lds.S

@@ -285,6 +285,15 @@ SECTIONS
 	}
 #endif
 
+#ifdef CONFIG_X86_KERNEL_IBT
+	. = ALIGN(8);
+	.ibt_endbr_seal : AT(ADDR(.ibt_endbr_seal) - LOAD_OFFSET) {
+		__ibt_endbr_seal = .;
+		*(.ibt_endbr_seal)
+		__ibt_endbr_seal_end = .;
+	}
+#endif
+
 	/*
 	 * struct alt_inst entries. From the header (alternative.h):
 	 * "Alternative instructions for different CPU types or capabilities"

tools/objtool/check.c

@@ -383,6 +383,7 @@ static int decode_instructions(struct objtool_file *file)
 			memset(insn, 0, sizeof(*insn));
 			INIT_LIST_HEAD(&insn->alts);
 			INIT_LIST_HEAD(&insn->stack_ops);
+			INIT_LIST_HEAD(&insn->call_node);
 
 			insn->sec = sec;
 			insn->offset = offset;
@@ -420,8 +421,9 @@ static int decode_instructions(struct objtool_file *file)
 
 			sym_for_each_insn(file, func, insn) {
 				insn->func = func;
-				if (insn->type == INSN_ENDBR) {
+				if (insn->type == INSN_ENDBR && list_empty(&insn->call_node)) {
 					if (insn->offset == insn->func->offset) {
+						list_add_tail(&insn->call_node, &file->endbr_list);
 						file->nr_endbr++;
 					} else {
 						file->nr_endbr_int++;
@@ -742,6 +744,58 @@ static int create_retpoline_sites_sections(struct objtool_file *file)
 	return 0;
 }
 
+static int create_ibt_endbr_seal_sections(struct objtool_file *file)
+{
+	struct instruction *insn;
+	struct section *sec;
+	int idx;
+
+	sec = find_section_by_name(file->elf, ".ibt_endbr_seal");
+	if (sec) {
+		WARN("file already has .ibt_endbr_seal, skipping");
+		return 0;
+	}
+
+	idx = 0;
+	list_for_each_entry(insn, &file->endbr_list, call_node)
+		idx++;
+
+	if (stats) {
+		printf("ibt: ENDBR at function start: %d\n", file->nr_endbr);
+		printf("ibt: ENDBR inside functions:  %d\n", file->nr_endbr_int);
+		printf("ibt: superfluous ENDBR:       %d\n", idx);
+	}
+
+	if (!idx)
+		return 0;
+
+	sec = elf_create_section(file->elf, ".ibt_endbr_seal", 0,
+				 sizeof(int), idx);
+	if (!sec) {
+		WARN("elf_create_section: .ibt_endbr_seal");
+		return -1;
+	}
+
+	idx = 0;
+	list_for_each_entry(insn, &file->endbr_list, call_node) {
+
+		int *site = (int *)sec->data->d_buf + idx;
+		*site = 0;
+
+		if (elf_add_reloc_to_insn(file->elf, sec,
+					  idx * sizeof(int),
+					  R_X86_64_PC32,
+					  insn->sec, insn->offset)) {
+			WARN("elf_add_reloc_to_insn: .ibt_endbr_seal");
+			return -1;
+		}
+
+		idx++;
+	}
+
+	return 0;
+}
+
 static int create_mcount_loc_sections(struct objtool_file *file)
 {
 	struct section *sec;
@@ -3120,8 +3174,12 @@ validate_ibt_reloc(struct objtool_file *file, struct reloc *reloc)
 	if (!dest)
 		return NULL;
 
-	if (dest->type == INSN_ENDBR)
+	if (dest->type == INSN_ENDBR) {
+		if (!list_empty(&dest->call_node))
+			list_del_init(&dest->call_node);
+
 		return NULL;
+	}
 
 	if (reloc->sym->static_call_tramp)
 		return NULL;
@@ -3860,6 +3918,13 @@ int check(struct objtool_file *file)
 		warnings += ret;
 	}
 
+	if (ibt) {
+		ret = create_ibt_endbr_seal_sections(file);
+		if (ret < 0)
+			goto out;
+		warnings += ret;
+	}
+
 	if (stats) {
 		printf("nr_insns_visited: %ld\n", nr_insns_visited);
 		printf("nr_cfi: %ld\n", nr_cfi);

tools/objtool/include/objtool/objtool.h

@@ -26,6 +26,7 @@ struct objtool_file {
 	struct list_head retpoline_call_list;
 	struct list_head static_call_list;
 	struct list_head mcount_loc_list;
+	struct list_head endbr_list;
 	bool ignore_unreachables, c_file, hints, rodata;
 
 	unsigned int nr_endbr;

tools/objtool/objtool.c

@@ -128,6 +128,7 @@ struct objtool_file *objtool_open_read(const char *_objname)
 	INIT_LIST_HEAD(&file.retpoline_call_list);
 	INIT_LIST_HEAD(&file.static_call_list);
 	INIT_LIST_HEAD(&file.mcount_loc_list);
+	INIT_LIST_HEAD(&file.endbr_list);
 	file.c_file = !vmlinux && find_section_by_name(file.elf, ".comment");
 	file.ignore_unreachables = no_unreachable;
 	file.hints = false;