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objtool: Find unused ENDBR instructions

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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
This commit is contained in:
Peter Zijlstra 2022-03-08 16:30:55 +01:00
parent 08f87a93c8
commit 89bc853eae
4 changed files with 78 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
arch/x86/kernel/vmlinux.lds.S
View File

@ -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"

69
tools/objtool/check.c
View File

@ -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);

1
tools/objtool/include/objtool/objtool.h
View File

@ -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;

1
tools/objtool/objtool.c
View File

@ -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;