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x86/ibt: Implement FineIBT 

Implement an alternative CFI scheme that merges both the fine-grained
nature of kCFI but also takes full advantage of the coarse grained
hardware CFI as provided by IBT.

To contrast:

  kCFI is a pure software CFI scheme and relies on being able to read
text -- specifically the instruction *before* the target symbol, and
does the hash validation *before* doing the call (otherwise control
flow is compromised already).

  FineIBT is a software and hardware hybrid scheme; by ensuring every
branch target starts with a hash validation it is possible to place
the hash validation after the branch. This has several advantages:

   o the (hash) load is avoided; no memop; no RX requirement.

   o IBT WAIT-FOR-ENDBR state is a speculation stop; by placing
     the hash validation in the immediate instruction after
     the branch target there is a minimal speculation window
     and the whole is a viable defence against SpectreBHB.

   o Kees feels obliged to mention it is slightly more vulnerable
     when the attacker can write code.

Obviously this patch relies on kCFI, but additionally it also relies
on the padding from the call-depth-tracking patches. It uses this
padding to place the hash-validation while the call-sites are
re-written to modify the indirect target to be 16 bytes in front of
the original target, thus hitting this new preamble.

Notably, there is no hardware that needs call-depth-tracking (Skylake)
and supports IBT (Tigerlake and onwards).

Suggested-by: Joao Moreira (Intel) <joao@overdrivepizza.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221027092842.634714496@infradead.org
This commit is contained in:
Peter Zijlstra 2022-10-27 11:28:14 +02:00
parent 9a479f766b
commit 931ab63664
11 changed files with 296 additions and 23 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
arch/um/kernel/um_arch.c
View File

@ -444,6 +444,11 @@ void apply_returns(s32 *start, s32 *end)
{
}
void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
s32 *start_cfi, s32 *end_cfi)
{
}
void apply_alternatives(struct alt_instr *start, struct alt_instr *end)
{
}

14
arch/x86/Kconfig
View File

@ -2463,17 +2463,27 @@ config FUNCTION_PADDING_BYTES
default FUNCTION_PADDING_CFI if CFI_CLANG
default FUNCTION_ALIGNMENT
config CALL_PADDING
def_bool n
depends on CC_HAS_ENTRY_PADDING && OBJTOOL
select FUNCTION_ALIGNMENT_16B
config FINEIBT
def_bool y
depends on X86_KERNEL_IBT && CFI_CLANG && RETPOLINE
select CALL_PADDING
config HAVE_CALL_THUNKS
def_bool y
depends on CC_HAS_ENTRY_PADDING && RETHUNK && OBJTOOL
config CALL_THUNKS
def_bool n
select FUNCTION_ALIGNMENT_16B
select CALL_PADDING
config PREFIX_SYMBOLS
def_bool y
depends on CALL_THUNKS && !CFI_CLANG
depends on CALL_PADDING && !CFI_CLANG
menuconfig SPECULATION_MITIGATIONS
bool "Mitigations for speculative execution vulnerabilities"

2
arch/x86/Makefile
View File

@ -208,7 +208,7 @@ ifdef CONFIG_SLS
KBUILD_CFLAGS += -mharden-sls=all
endif
ifdef CONFIG_CALL_THUNKS
ifdef CONFIG_CALL_PADDING
PADDING_CFLAGS := -fpatchable-function-entry=$(CONFIG_FUNCTION_PADDING_BYTES),$(CONFIG_FUNCTION_PADDING_BYTES)
KBUILD_CFLAGS += $(PADDING_CFLAGS)
export PADDING_CFLAGS

2
arch/x86/include/asm/alternative.h
View File

@ -78,6 +78,8 @@ extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
extern void apply_retpolines(s32 *start, s32 *end);
extern void apply_returns(s32 *start, s32 *end);
extern void apply_ibt_endbr(s32 *start, s32 *end);
extern void apply_fineibt(s32 *start_retpoline, s32 *end_retpoine,
s32 *start_cfi, s32 *end_cfi);
struct module;
struct paravirt_patch_site;

6
arch/x86/include/asm/linkage.h
View File

@ -15,7 +15,7 @@
#define __ALIGN .balign CONFIG_FUNCTION_ALIGNMENT, 0x90;
#define __ALIGN_STR __stringify(__ALIGN)
#if defined(CONFIG_CALL_THUNKS) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
#if defined(CONFIG_CALL_PADDING) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
#define FUNCTION_PADDING .skip CONFIG_FUNCTION_ALIGNMENT, 0x90;
#else
#define FUNCTION_PADDING
@ -57,7 +57,7 @@
#endif /* __ASSEMBLY__ */
/*
* Depending on -fpatchable-function-entry=N,N usage (CONFIG_CALL_THUNKS) the
* Depending on -fpatchable-function-entry=N,N usage (CONFIG_CALL_PADDING) the
* CFI symbol layout changes.
*
* Without CALL_THUNKS:
@ -81,7 +81,7 @@
* In both cases the whole thing is FUNCTION_ALIGNMENT aligned and sized.
*/
#ifdef CONFIG_CALL_THUNKS
#ifdef CONFIG_CALL_PADDING
#define CFI_PRE_PADDING
#define CFI_POST_PADDING .skip CONFIG_FUNCTION_PADDING_BYTES, 0x90;
#else

257
arch/x86/kernel/alternative.c
View File

@ -116,6 +116,7 @@ static void __init_or_module add_nops(void *insns, unsigned int len)
extern s32 __retpoline_sites[], __retpoline_sites_end[];
extern s32 __return_sites[], __return_sites_end[];
extern s32 __cfi_sites[], __cfi_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[];
@ -656,6 +657,28 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end) { }
#ifdef CONFIG_X86_KERNEL_IBT
static void poison_endbr(void *addr, bool warn)
{
u32 endbr, poison = gen_endbr_poison();
if (WARN_ON_ONCE(get_kernel_nofault(endbr, addr)))
return;
if (!is_endbr(endbr)) {
WARN_ON_ONCE(warn);
return;
}
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);
}
/*
* Generated by: objtool --ibt
*/
@ -664,23 +687,11 @@ 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);
poison_endbr(addr, true);
if (IS_ENABLED(CONFIG_FINEIBT))
poison_endbr(addr - 16, false);
}
}
@ -690,6 +701,219 @@ void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end) { }
#endif /* CONFIG_X86_KERNEL_IBT */
#ifdef CONFIG_FINEIBT
/*
* kCFI FineIBT
*
* __cfi_\func: __cfi_\func:
* movl $0x12345678,%eax // 5 endbr64 // 4
* nop subl $0x12345678,%r10d // 7
* nop jz 1f // 2
* nop ud2 // 2
* nop 1: nop // 1
* nop
* nop
* nop
* nop
* nop
* nop
* nop
*
*
* caller: caller:
* movl $(-0x12345678),%r10d // 6 movl $0x12345678,%r10d // 6
* addl $-15(%r11),%r10d // 4 sub $16,%r11 // 4
* je 1f // 2 nop4 // 4
* ud2 // 2
* 1: call __x86_indirect_thunk_r11 // 5 call *%r11; nop2; // 5
*
*/
asm( ".pushsection .rodata \n"
"fineibt_preamble_start: \n"
" endbr64 \n"
" subl $0x12345678, %r10d \n"
" je fineibt_preamble_end \n"
" ud2 \n"
" nop \n"
"fineibt_preamble_end: \n"
".popsection\n"
);
extern u8 fineibt_preamble_start[];
extern u8 fineibt_preamble_end[];
#define fineibt_preamble_size (fineibt_preamble_end - fineibt_preamble_start)
#define fineibt_preamble_hash 7
asm( ".pushsection .rodata \n"
"fineibt_caller_start: \n"
" movl $0x12345678, %r10d \n"
" sub $16, %r11 \n"
ASM_NOP4
"fineibt_caller_end: \n"
".popsection \n"
);
extern u8 fineibt_caller_start[];
extern u8 fineibt_caller_end[];
#define fineibt_caller_size (fineibt_caller_end - fineibt_caller_start)
#define fineibt_caller_hash 2
#define fineibt_caller_jmp (fineibt_caller_size - 2)
static u32 decode_preamble_hash(void *addr)
{
u8 *p = addr;
/* b8 78 56 34 12 mov $0x12345678,%eax */
if (p[0] == 0xb8)
return *(u32 *)(addr + 1);
return 0; /* invalid hash value */
}
static u32 decode_caller_hash(void *addr)
{
u8 *p = addr;
/* 41 ba 78 56 34 12 mov $0x12345678,%r10d */
if (p[0] == 0x41 && p[1] == 0xba)
return -*(u32 *)(addr + 2);
/* e8 0c 78 56 34 12 jmp.d8 +12 */
if (p[0] == JMP8_INSN_OPCODE && p[1] == fineibt_caller_jmp)
return -*(u32 *)(addr + 2);
return 0; /* invalid hash value */
}
/* .retpoline_sites */
static int cfi_disable_callers(s32 *start, s32 *end)
{
/*
* Disable kCFI by patching in a JMP.d8, this leaves the hash immediate
* in tact for later usage. Also see decode_caller_hash() and
* cfi_rewrite_callers().
*/
const u8 jmp[] = { JMP8_INSN_OPCODE, fineibt_caller_jmp };
s32 *s;
for (s = start; s < end; s++) {
void *addr = (void *)s + *s;
u32 hash;
addr -= fineibt_caller_size;
hash = decode_caller_hash(addr);
if (!hash) /* nocfi callers */
continue;
text_poke_early(addr, jmp, 2);
}
return 0;
}
/* .cfi_sites */
static int cfi_rewrite_preamble(s32 *start, s32 *end)
{
s32 *s;
for (s = start; s < end; s++) {
void *addr = (void *)s + *s;
u32 hash;
hash = decode_preamble_hash(addr);
if (WARN(!hash, "no CFI hash found at: %pS %px %*ph\n",
addr, addr, 5, addr))
return -EINVAL;
text_poke_early(addr, fineibt_preamble_start, fineibt_preamble_size);
WARN_ON(*(u32 *)(addr + fineibt_preamble_hash) != 0x12345678);
text_poke_early(addr + fineibt_preamble_hash, &hash, 4);
}
return 0;
}
/* .retpoline_sites */
static int cfi_rewrite_callers(s32 *start, s32 *end)
{
s32 *s;
for (s = start; s < end; s++) {
void *addr = (void *)s + *s;
u32 hash;
addr -= fineibt_caller_size;
hash = decode_caller_hash(addr);
if (hash) {
text_poke_early(addr, fineibt_caller_start, fineibt_caller_size);
WARN_ON(*(u32 *)(addr + fineibt_caller_hash) != 0x12345678);
text_poke_early(addr + fineibt_caller_hash, &hash, 4);
}
/* rely on apply_retpolines() */
}
return 0;
}
static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
s32 *start_cfi, s32 *end_cfi, bool builtin)
{
int ret;
if (WARN_ONCE(fineibt_preamble_size != 16,
"FineIBT preamble wrong size: %ld", fineibt_preamble_size))
return;
if (!HAS_KERNEL_IBT || !cpu_feature_enabled(X86_FEATURE_IBT))
return;
/*
* Rewrite the callers to not use the __cfi_ stubs, such that we might
* rewrite them. This disables all CFI. If this succeeds but any of the
* later stages fails, we're without CFI.
*/
ret = cfi_disable_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
ret = cfi_rewrite_preamble(start_cfi, end_cfi);
if (ret)
goto err;
ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
if (builtin)
pr_info("Using FineIBT CFI\n");
return;
err:
pr_err("Something went horribly wrong trying to rewrite the CFI implementation.\n");
}
#else
static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
s32 *start_cfi, s32 *end_cfi, bool builtin)
{
}
#endif
void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
s32 *start_cfi, s32 *end_cfi)
{
return __apply_fineibt(start_retpoline, end_retpoline,
start_cfi, end_cfi,
/* .builtin = */ false);
}
#ifdef CONFIG_SMP
static void alternatives_smp_lock(const s32 *start, const s32 *end,
u8 *text, u8 *text_end)
@ -996,6 +1220,9 @@ void __init alternative_instructions(void)
*/
apply_paravirt(__parainstructions, __parainstructions_end);
__apply_fineibt(__retpoline_sites, __retpoline_sites_end,
__cfi_sites, __cfi_sites_end, true);
/*
* Rewrite the retpolines, must be done before alternatives since
* those can rewrite the retpoline thunks.

1
arch/x86/kernel/cpu/common.c
View File

@ -609,6 +609,7 @@ static __always_inline void setup_cet(struct cpuinfo_x86 *c)
if (!ibt_selftest()) {
pr_err("IBT selftest: Failed!\n");
wrmsrl(MSR_IA32_S_CET, 0);
setup_clear_cpu_cap(X86_FEATURE_IBT);
return;
}

20
arch/x86/kernel/module.c
View File

@ -255,7 +255,7 @@ 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, *returns = NULL, *ibt_endbr = NULL,
*calls = NULL;
*calls = NULL, *cfi = NULL;
char *secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
@ -277,6 +277,8 @@ int module_finalize(const Elf_Ehdr *hdr,
returns = s;
if (!strcmp(".call_sites", secstrings + s->sh_name))
calls = s;
if (!strcmp(".cfi_sites", secstrings + s->sh_name))
cfi = s;
if (!strcmp(".ibt_endbr_seal", secstrings + s->sh_name))
ibt_endbr = s;
}
@ -289,6 +291,22 @@ int module_finalize(const Elf_Ehdr *hdr,
void *pseg = (void *)para->sh_addr;
apply_paravirt(pseg, pseg + para->sh_size);
}
if (retpolines || cfi) {
void *rseg = NULL, *cseg = NULL;
unsigned int rsize = 0, csize = 0;
if (retpolines) {
rseg = (void *)retpolines->sh_addr;
rsize = retpolines->sh_size;
}
if (cfi) {
cseg = (void *)cfi->sh_addr;
csize = cfi->sh_size;
}
apply_fineibt(rseg, rseg + rsize, cseg, cseg + csize);
}
if (retpolines) {
void *rseg = (void *)retpolines->sh_addr;
apply_retpolines(rseg, rseg + retpolines->sh_size);

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

@ -309,6 +309,15 @@ SECTIONS
}
#endif
#ifdef CONFIG_FINEIBT
. = ALIGN(8);
.cfi_sites : AT(ADDR(.cfi_sites) - LOAD_OFFSET) {
__cfi_sites = .;
*(.cfi_sites)
__cfi_sites_end = .;
}
#endif
/*
* struct alt_inst entries. From the header (alternative.h):
* "Alternative instructions for different CPU types or capabilities"

2
include/linux/bpf.h
View File

@ -984,7 +984,7 @@ int arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int num_func
}
#ifdef CONFIG_X86_64
#ifdef CONFIG_CALL_THUNKS
#ifdef CONFIG_CALL_PADDING
#define BPF_DISPATCHER_ATTRIBUTES __attribute__((patchable_function_entry(5+CONFIG_FUNCTION_PADDING_BYTES,CONFIG_FUNCTION_PADDING_BYTES)))
#else
#define BPF_DISPATCHER_ATTRIBUTES __attribute__((patchable_function_entry(5)))

1
scripts/Makefile.lib
View File

@ -256,6 +256,7 @@ objtool-args-$(CONFIG_HAVE_JUMP_LABEL_HACK) += --hacks=jump_label
objtool-args-$(CONFIG_HAVE_NOINSTR_HACK) += --hacks=noinstr
objtool-args-$(CONFIG_CALL_DEPTH_TRACKING) += --hacks=skylake
objtool-args-$(CONFIG_X86_KERNEL_IBT) += --ibt
objtool-args-$(CONFIG_FINEIBT) += --cfi
objtool-args-$(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL) += --mcount
objtool-args-$(CONFIG_UNWINDER_ORC) += --orc
objtool-args-$(CONFIG_RETPOLINE) += --retpoline