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Date: Fri, 20 May 2022 14:49:06 +0100 From: Matthew Wilcox <willy@...radead.org> To: Peter Zijlstra <peterz@...radead.org> Cc: Sami Tolvanen <samitolvanen@...gle.com>, linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Joao Moreira <joao@...rdrivepizza.com>, Sedat Dilek <sedat.dilek@...il.com>, Steven Rostedt <rostedt@...dmis.org>, linux-hardening@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG 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; >
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