Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

Matthew Wilcox
2022-05-20T13:49:18Z

On Mon, May 16, 2022 at 02:58:44PM +0200, Peter Zijlstra wrote:
> @willy, how horribly broken is this xarray usage?

The xarray doesn't work very well as a hash ;-( It has pretty much
pessimal memory usage if you have a good hash. You'll end up allocating
essentially the entire 4 billion * ptr_size address space of the hash.
Can you use an rhashtable instead?

> ---
> arch/x86/include/asm/traps.h | 1 +
> arch/x86/kernel/alternative.c | 316 ++++++++++++++++++++++++++++++++
> arch/x86/kernel/cpu/common.c | 5 +
> arch/x86/kernel/vmlinux.lds.S | 9 +
> tools/objtool/check.c | 67 ++++++-
> tools/objtool/include/objtool/objtool.h | 1 +
> tools/objtool/objtool.c | 1 +
> 7 files changed, 399 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index 35317c5c551d..a423343cffbc 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -19,6 +19,7 @@ asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *e
> #endif
>
> extern bool ibt_selftest(void);
> +extern bool ibt_broken;
>
> #ifdef CONFIG_X86_F00F_BUG
> /* For handling the FOOF bug */
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index d374cb3cf024..abce4e78a1e0 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -18,6 +18,9 @@
> #include <linux/mmu_context.h>
> #include <linux/bsearch.h>
> #include <linux/sync_core.h>
> +#include <linux/moduleloader.h>
> +#include <linux/xarray.h>
> +#include <linux/set_memory.h>
> #include <asm/text-patching.h>
> #include <asm/alternative.h>
> #include <asm/sections.h>
> @@ -115,6 +118,7 @@ static void __init_or_module add_nops(void *insns, unsigned int len)
> }
>
> extern s32 __retpoline_sites[], __retpoline_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[];
> @@ -549,6 +553,315 @@ void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end) { }
>
> #endif /* CONFIG_X86_KERNEL_IBT */
>
> +#ifdef CONFIG_CFI_CLANG
> +/*
> + * FineIBT kCFI
> + *
> + * __fineibt_\hash:
> + * xor \hash, %r10 # 7
> + * jz 1f # 2
> + * ud2 # 2
> + * 1:ret # 1
> + * int3 # 1
> + *
> + *
> + * __cfi_\sym: __cfi_\sym:
> + * int3; int3 # 2
> + * endbr # 4 mov \hash, %eax # 5
> + * call __fineibt_\hash # 5 int3; int3 # 2
> + * \sym: \sym:
> + * ... ...
> + *
> + *
> + * caller: caller:
> + * movl \hash, %r10d # 6 cmpl \hash, -6(%r11) # 8
> + * sub $9, %r11 # 4 je 1f # 2
> + * nop2 # 2 ud2 # 2
> + *
> + * call *%r11 # 3 call __x86_indirect_thunk_r11 # 5
> + * nop2 # 2
> + */
> +
> +static DEFINE_XARRAY(cfi_hashes);
> +static int nr_cfi_hashes;
> +
> +static u32 decode_cfi_preamble(void *addr)
> +{
> + u8 *p = addr;
> +
> + if (p[0] == 0xcc && p[1] == 0xcc &&
> + p[2] == 0xb8 &&
> + p[7] == 0xcc && p[8] == 0xcc)
> + return *(u32 *)(addr + 3);
> +
> + return 0; /* invalid hash value */
> +}
> +
> +static u32 decode_cfi_caller(void *addr)
> +{
> + u8 *p = addr;
> +
> + if (((p[0] == 0x41 && p[1] == 0x81) ||
> + (p[0] == 0xeb && p[1] == 0x0a)) && p[2] == 0x7b &&
> + p[8] == 0x74 && p[9] == 0x02 &&
> + p[10] == 0x0f && p[11] == 0x0b)
> + return *(u32 *)(addr + 4);
> +
> + return 0; /* invalid hash value */
> +}
> +
> +// .cfi_sites
> +static int cfi_index_hashes(s32 *start, s32 *end)
> +{
> + s32 *s;
> +
> + for (s = start; s < end; s++) {
> + void *addr = (void *)s + *s;
> + void *xa;
> + u32 hash;
> +
> + hash = decode_cfi_preamble(addr);
> + if (!hash) {
> + //WARN();
> + return -EINVAL;
> + }
> +
> + xa = xa_store(&cfi_hashes, hash, NULL, GFP_KERNEL);
> + if (xa_is_err(xa)) {
> + //WARN();
> + return xa_err(xa);
> + }
> + nr_cfi_hashes++;
> + }
> +
> + return 0;
> +}
> +
> +asm ( ".pushsection .rodata\n"
> + "fineibt_template_start:\n"
> + " xorl $0x12345678, %r10d\n" // 7
> + " je 1f\n" // 2
> + " ud2\n" // 2
> + "1: ret\n" // 1
> + " int3\n"
> + " int3\n"
> + " int3\n"
> + " int3\n" // 4
> + "fineibt_template_end:\n"
> + ".popsection\n"
> + );
> +
> +extern u8 fineibt_template_start[];
> +extern u8 fineibt_template_end[];
> +
> +static int cfi_create_fineibt_stubs(void)
> +{
> + size_t size = 16 * nr_cfi_hashes;
> + int pages = 1 + ((size - 1) >> PAGE_SHIFT);
> + void *text, *entry, *xa;
> + unsigned long hash;
> + int err = -ENOMEM;
> +
> + text = module_alloc(size);
> + if (!text)
> + return err;
> +
> + entry = text;
> + xa_for_each(&cfi_hashes, hash, xa) {
> +
> + memcpy(entry, fineibt_template_start, 16);
> + *(u32 *)(entry + 3) = hash;
> +
> + xa = xa_store(&cfi_hashes, hash, entry, GFP_KERNEL);
> + if (xa_is_err(xa)) {
> + err = xa_err(xa);
> + goto err_alloc;
> + }
> + if (xa) {
> + err = -EINVAL;
> + goto err_alloc;
> + }
> +
> + entry += 16;
> + }
> +
> + set_memory_ro((unsigned long)text, pages);
> + set_memory_x((unsigned long)text, pages);
> +
> + return 0;
> +
> +err_alloc:
> + module_memfree(text);
> + return -EINVAL;
> +}
> +
> +// .retpoline_sites
> +static int cfi_disable_callers(s32 *start, s32 *end)
> +{
> + /*
> + * Disable CFI by patching in a 2 byte JMP, this leaves the hash in
> + * tact for later usage. Also see decode_cfi_caller() and
> + * cfu_rewrite_callers().
> + */
> + const u8 jmp12[] = { 0xeb, 0x0a };
> + s32 *s;
> +
> + for (s = start; s < end; s++) {
> + void *addr = (void *)s + *s;
> + u32 hash;
> +
> + hash = decode_cfi_caller(addr - 12);
> + if (!hash) {
> + // WARN();
> + return -EINVAL;
> + }
> +
> + text_poke_early(addr - 12, jmp12, 2);
> + }
> +
> + return 0;
> +}
> +
> +asm ( ".pushsection .rodata\n"
> + "fineibt_cfi_start:\n"
> + " endbr64\n"
> + " call fineibt_caller_start\n"
> + "fineibt_cfi_end:"
> + ".popsection\n"
> + );
> +
> +extern u8 fineibt_cfi_start[];
> +extern u8 fineibt_cfi_end[];
> +
> +// .cfi_sites
> +static int cfi_rewrite_cfi(s32 *start, s32 *end)
> +{
> + s32 *s;
> +
> + for (s = start; s < end; s++) {
> + void *dest, *addr = (void *)s + *s;
> + unsigned long index;
> + u32 hash;
> +
> + index = hash = decode_cfi_preamble(addr);
> + dest = xa_find(&cfi_hashes, &index, hash, XA_PRESENT);
> +
> + if (WARN_ON_ONCE(index != hash || !dest))
> + return -EINVAL;
> +
> + text_poke_early(addr, fineibt_cfi_start,
> + (fineibt_cfi_end - fineibt_cfi_start));
> +
> + __text_gen_insn(addr + 4,
> + CALL_INSN_OPCODE, addr + 4,
> + dest, CALL_INSN_SIZE);
> + }
> +
> + return 0;
> +}
> +
> +asm ( ".pushsection .rodata\n"
> + "fineibt_caller_start:\n"
> + " movl $0x12345678, %r10d\n"
> + " sub $9, %r11\n"
> + " .nops 2\n"
> + "fineibt_caller_end:"
> + ".popsection\n"
> + );
> +
> +extern u8 fineibt_caller_start[];
> +extern u8 fineibt_caller_end[];
> +
> +// .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;
> +
> + hash = decode_cfi_caller(addr - 12);
> +
> + if (WARN_ON_ONCE(!hash))
> + return -EINVAL;
> +
> + text_poke_early(addr - 12, fineibt_caller_start,
> + (fineibt_caller_end - fineibt_caller_end));
> +
> + *(u32 *)(addr - 12 + 2) = hash;
> +
> + /* rely on apply_retpolines() to rewrite the actual call */
> + }
> +
> + return 0;
> +}
> +
> +bool __ro_after_init ibt_broken = false;
> +
> +static void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
> + s32 *start_cfi, s32 *end_cfi)
> +{
> + int ret;
> +
> + /* If IBT, use FineIBT */
> + if (!HAS_KERNEL_IBT || !cpu_feature_enabled(X86_FEATURE_IBT))
> + return;
> +
> + /*
> + * Find and count all unique hash values.
> + */
> + ret = cfi_index_hashes(start_cfi, end_cfi);
> + if (ret)
> + goto err;
> +
> + /*
> + * Allocate module memory and write FineIBT stubs.
> + */
> + ret = cfi_create_fineibt_stubs();
> + if (ret)
> + goto err;
> +
> + /*
> + * Rewrite the callers to not use the __cfi_ stubs, such that we might
> + * rewrite them. Disables all CFI. If this succeeds but any of the
> + * later stages fails, we're CFI-less.
> + */
> + ret = cfi_disable_callers(start_retpoline, end_retpoline);
> + if (ret)
> + goto err;
> +
> + /*
> + * Rewrite the __cfi_ stubs from kCFI to FineIBT.
> + */
> + ret = cfi_rewrite_cfi(start_cfi, end_cfi);
> + if (ret)
> + goto err;
> +
> + /*
> + * Now that everything is in place; rewrite the callers to FineIBT.
> + */
> + ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
> + if (ret)
> + goto err;
> +
> + return;
> +
> +err:
> + pr_err("Something went horribly wrong trying to rewrite the CFI implementation.\n");
> + /* must *NOT* enable IBT */
> + ibt_broken = true;
> +}
> +
> +#else
> +
> +static void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
> + s32 *start_cfi, s32 *end_cfi)
> +{
> +}
> +
> +#endif
> +
> #ifdef CONFIG_SMP
> static void alternatives_smp_lock(const s32 *start, const s32 *end,
> u8 *text, u8 *text_end)
> @@ -855,6 +1168,9 @@ void __init alternative_instructions(void)
> */
> apply_paravirt(__parainstructions, __parainstructions_end);
>
> + apply_fineibt(__retpoline_sites, __retpoline_sites_end,
> + __cfi_sites, __cfi_sites_end);
> +
> /*
> * Rewrite the retpolines, must be done before alternatives since
> * those can rewrite the retpoline thunks.
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index e342ae4db3c4..e4377256b952 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -630,6 +630,11 @@ static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> !cpu_feature_enabled(X86_FEATURE_IBT))
> return;
>
> +#ifdef CONFIG_CFI_CLANG
> + if (ibt_broken)
> + return;
> +#endif
> +
> wrmsrl(MSR_IA32_S_CET, msr);
> cr4_set_bits(X86_CR4_CET);
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 7fda7f27e762..72ffc91ddd20 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -294,6 +294,15 @@ SECTIONS
> }
> #endif
>
> +#if defined(CONFIG_CFI_CLANG) && defined(CONFIG_RETPOLINE) && defined(CONFIG_X86_KERNEL_IBT)
> + . = 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"
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 88f005ae6dcc..edc8aecf229c 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -797,6 +797,52 @@ static int create_ibt_endbr_seal_sections(struct objtool_file *file)
> return 0;
> }
>
> +static int create_cfi_sections(struct objtool_file *file)
> +{
> + struct instruction *insn;
> + struct section *sec;
> + int idx;
> +
> + sec = find_section_by_name(file->elf, ".cfi_sites");
> + if (sec) {
> + WARN("file already has .cfi_sites, skipping");
> + return 0;
> + }
> +
> + idx = 0;
> + list_for_each_entry(insn, &file->cfi_list, call_node)
> + idx++;
> +
> + if (!idx)
> + return 0;
> +
> + sec = elf_create_section(file->elf, ".cfi_sites", 0,
> + sizeof(int), idx);
> + if (!sec) {
> + WARN("elf_create_section: .cfi_sites");
> + return -1;
> + }
> +
> + idx = 0;
> + list_for_each_entry(insn, &file->cfi_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: .cfi_sites");
> + return -1;
> + }
> +
> + idx++;
> + }
> +
> + return 0;
> +}
> +
> static int create_mcount_loc_sections(struct objtool_file *file)
> {
> struct section *sec;
> @@ -3301,6 +3347,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
> {
> struct alternative *alt;
> struct instruction *next_insn, *prev_insn = NULL;
> + struct instruction *first_insn = insn;
> struct section *sec;
> u8 visited;
> int ret;
> @@ -3312,8 +3359,19 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>
> if (func && insn->func && func != insn->func->pfunc) {
> /* Ignore KCFI type preambles, which always fall through */
> - if (!strncmp(func->name, "__cfi_", 6))
> + if (!strncmp(func->name, "__cfi_", 6)) {
> + /*
> + * If the function has a __cfi_ preamble, the endbr
> + * will live in there.
> + */
> + insn->noendbr = true;
> + /*
> + * The preamble starts with INSN_TRAP,
> + * call_node cannot be used.
> + */
> + list_add_tail(&first_insn->call_node, &file->cfi_list);
> return 0;
> + }
>
> WARN("%s() falls through to next function %s()",
> func->name, insn->func->name);
> @@ -3953,6 +4011,13 @@ int check(struct objtool_file *file)
> warnings += ret;
> }
>
> + if (ibt && retpoline) {
> + ret = create_cfi_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);
> diff --git a/tools/objtool/include/objtool/objtool.h b/tools/objtool/include/objtool/objtool.h
> index a6e72d916807..93f52e275fa6 100644
> --- a/tools/objtool/include/objtool/objtool.h
> +++ b/tools/objtool/include/objtool/objtool.h
> @@ -27,6 +27,7 @@ struct objtool_file {
> struct list_head static_call_list;
> struct list_head mcount_loc_list;
> struct list_head endbr_list;
> + struct list_head cfi_list;
> bool ignore_unreachables, hints, rodata;
>
> unsigned int nr_endbr;
> diff --git a/tools/objtool/objtool.c b/tools/objtool/objtool.c
> index 843ff3c2f28e..16ed3613b0e2 100644
> --- a/tools/objtool/objtool.c
> +++ b/tools/objtool/objtool.c
> @@ -129,6 +129,7 @@ struct objtool_file *objtool_open_read(const char *_objname)
> INIT_LIST_HEAD(&file.static_call_list);
> INIT_LIST_HEAD(&file.mcount_loc_list);
> INIT_LIST_HEAD(&file.endbr_list);
> + INIT_LIST_HEAD(&file.cfi_list);
> file.ignore_unreachables = no_unreachable;
> file.hints = false;
>