[PATCH v3 39/39] x86/alternative: Use .ibt_endbr_seal to seal indirect calls

Peter Zijlstra
2022-03-03T11:33:01Z

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

Overwrite these ENDBR instructions with a NOP4 such that these
function can never be indirect called, reducing the number of viable
ENDBR targets in the kernel.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/um/kernel/um_arch.c | 4 +++
arch/x86/Kconfig | 9 +++++++-
arch/x86/include/asm/alternative.h | 1
arch/x86/include/asm/ibt.h | 12 +++++++++++
arch/x86/kernel/alternative.c | 39 +++++++++++++++++++++++++++++++++++++
arch/x86/kernel/module.c | 8 ++++++-
scripts/Makefile.lib | 11 ++++++++++
scripts/link-vmlinux.sh | 10 +++++++--
8 files changed, 90 insertions(+), 4 deletions(-)

--- 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
@@ -1873,7 +1873,7 @@ config CC_HAS_IBT
config X86_KERNEL_IBT
prompt "Indirect Branch Tracking"
bool
- depends on X86_64 && CC_HAS_IBT
+ depends on X86_64 && CC_HAS_IBT && STACK_VALIDATION
help
Build the kernel with support for Indirect Branch Tracking, a
hardware support course-grain forward-edge Control Flow Integrity
@@ -1881,6 +1881,13 @@ config X86_KERNEL_IBT
an ENDBR instruction, as such, the compiler will instrument the
code with them to make this happen.

+ 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.
+
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
@@ -33,8 +33,20 @@ static inline __attribute_const__ u32 ge
return endbr;
}

+static inline __attribute_const__ u32 gen_endbr_poison(void)
+{
+ /*
+ * 4 byte NOP that isn't NOP4 (in fact it is OSP NOP3), such that it
+ * will be unique to (former) ENDBR sites.
+ */
+ return 0x001f0f66; /* osp nopl (%rax) */
+}
+
static inline bool is_endbr(u32 val)
{
+ if (val == gen_endbr_poison())
+ return true;
+
val &= ~0x01000000U; /* ENDBR32 -> ENDBR64 */
return val == gen_endbr();
}
--- 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_seal[], __ibt_endbr_seal_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 +513,42 @@ void __init_or_module noinline apply_ret

#endif /* CONFIG_RETPOLINE && CONFIG_STACK_VALIDATION */

+#ifdef CONFIG_X86_KERNEL_IBT
+
+/*
+ * Generated by: objtool --ibt
+ */
+void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end)
+{
+ s32 *s;
+
+ for (s = start; s < end; s++) {
+ u32 endbr, poison = gen_endbr_poison();
+ void *addr = (void *)s + *s;
+
+ 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
+ */
+ DUMP_BYTES(((u8*)addr), 4, "%px: orig: ", addr);
+ DUMP_BYTES(((u8*)&poison), 4, "%px: repl: ", addr);
+ text_poke_early(addr, &poison, 4);
+ }
+}
+
+#else
+
+void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end) { }
+
+#endif /* CONFIG_X86_KERNEL_IBT */
+
#ifdef CONFIG_SMP
static void alternatives_smp_lock(const s32 *start, const s32 *end,
u8 *text, u8 *text_end)
@@ -833,6 +870,8 @@ void __init alternative_instructions(voi
*/
apply_alternatives(__alt_instructions, __alt_instructions_end);

+ apply_ibt_endbr(__ibt_endbr_seal, __ibt_endbr_seal_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
@@ -253,7 +253,7 @@ 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++) {
@@ -271,6 +271,8 @@ int module_finalize(const Elf_Ehdr *hdr,
orc_ip = s;
if (!strcmp(".retpoline_sites", secstrings + s->sh_name))
retpolines = s;
+ if (!strcmp(".ibt_endbr_seal", secstrings + s->sh_name))
+ ibt_endbr = s;
}

/*
@@ -290,6 +292,10 @@ 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/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -544,6 +544,7 @@ objtool := $(objtree)/tools/objtool/objt
objtool_args = \
$(if $(CONFIG_UNWINDER_ORC),orc generate,check) \
$(if $(part-of-module), --module) \
+ $(if $(CONFIG_X86_KERNEL_IBT), --lto --ibt) \
$(if $(CONFIG_FRAME_POINTER),, --no-fp) \
$(if $(CONFIG_GCOV_KERNEL)$(CONFIG_LTO_CLANG), --no-unreachable)\
$(if $(CONFIG_RETPOLINE), --retpoline) \
@@ -567,6 +568,16 @@ $(obj)/%.o: objtool-enabled :=
$(obj)/%.lto.o: objtool-enabled = y
$(obj)/%.lto.o: part-of-module := y

+else ifdef CONFIG_X86_KERNEL_IBT
+
+# Skip objtool on individual files
+$(obj)/%.o: objtool-enabled :=
+
+# instead run objtool on the module as a whole, right before
+# the final link pass with the linker script.
+%.ko: objtool-enabled = y
+%.ko: part-of-module := y
+
else

# 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory
--- 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_KERNEL_IBT ); 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_KERNEL_IBT; then
+ objtoolopt="${objtoolopt} --ibt"
+ fi
+
if is_enabled CONFIG_FTRACE_MCOUNT_USE_OBJTOOL; then
objtoolopt="${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_KERNEL_IBT; then
# Use vmlinux.o instead of performing the slow LTO link again.
objs=vmlinux.o
libs=