From 89245600941e4e0f87d77f60ee269b5e61ef4e49 Mon Sep 17 00:00:00 2001 From: Sami Tolvanen Date: Thu, 8 Sep 2022 14:54:47 -0700 Subject: [PATCH] cfi: Switch to -fsanitize=kcfi Switch from Clang's original forward-edge control-flow integrity implementation to -fsanitize=kcfi, which is better suited for the kernel, as it doesn't require LTO, doesn't use a jump table that requires altering function references, and won't break cross-module function address equality. Signed-off-by: Sami Tolvanen Reviewed-by: Kees Cook Tested-by: Kees Cook Tested-by: Nathan Chancellor Acked-by: Peter Zijlstra (Intel) Tested-by: Peter Zijlstra (Intel) Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20220908215504.3686827-6-samitolvanen@google.com --- Makefile | 13 +-- arch/Kconfig | 8 +- include/asm-generic/vmlinux.lds.h | 37 +++---- include/linux/cfi.h | 29 +++++- include/linux/compiler-clang.h | 14 +-- include/linux/module.h | 6 +- kernel/cfi.c | 156 +++++++++++++++--------------- kernel/module/main.c | 35 +------ scripts/module.lds.S | 23 +---- 9 files changed, 139 insertions(+), 182 deletions(-) diff --git a/Makefile b/Makefile index a4f71076cacb..43e08c9f95e9 100644 --- a/Makefile +++ b/Makefile @@ -921,18 +921,7 @@ export CC_FLAGS_LTO endif ifdef CONFIG_CFI_CLANG -CC_FLAGS_CFI := -fsanitize=cfi \ - -fsanitize-cfi-cross-dso \ - -fno-sanitize-cfi-canonical-jump-tables \ - -fno-sanitize-trap=cfi \ - -fno-sanitize-blacklist - -ifdef CONFIG_CFI_PERMISSIVE -CC_FLAGS_CFI += -fsanitize-recover=cfi -endif - -# If LTO flags are filtered out, we must also filter out CFI. -CC_FLAGS_LTO += $(CC_FLAGS_CFI) +CC_FLAGS_CFI := -fsanitize=kcfi KBUILD_CFLAGS += $(CC_FLAGS_CFI) export CC_FLAGS_CFI endif diff --git a/arch/Kconfig b/arch/Kconfig index 5fd875e18c99..1c1eca0c0019 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -738,11 +738,13 @@ config ARCH_SUPPORTS_CFI_CLANG An architecture should select this option if it can support Clang's Control-Flow Integrity (CFI) checking. +config ARCH_USES_CFI_TRAPS + bool + config CFI_CLANG bool "Use Clang's Control Flow Integrity (CFI)" - depends on LTO_CLANG && ARCH_SUPPORTS_CFI_CLANG - depends on CLANG_VERSION >= 140000 - select KALLSYMS + depends on ARCH_SUPPORTS_CFI_CLANG + depends on $(cc-option,-fsanitize=kcfi) help This option enables Clang’s forward-edge Control Flow Integrity (CFI) checking, where the compiler injects a runtime check to each diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index 7515a465ec03..7501edfce11e 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -421,6 +421,22 @@ __end_ro_after_init = .; #endif +/* + * .kcfi_traps contains a list KCFI trap locations. + */ +#ifndef KCFI_TRAPS +#ifdef CONFIG_ARCH_USES_CFI_TRAPS +#define KCFI_TRAPS \ + __kcfi_traps : AT(ADDR(__kcfi_traps) - LOAD_OFFSET) { \ + __start___kcfi_traps = .; \ + KEEP(*(.kcfi_traps)) \ + __stop___kcfi_traps = .; \ + } +#else +#define KCFI_TRAPS +#endif +#endif + /* * Read only Data */ @@ -529,6 +545,8 @@ __stop___modver = .; \ } \ \ + KCFI_TRAPS \ + \ RO_EXCEPTION_TABLE \ NOTES \ BTF \ @@ -537,21 +555,6 @@ __end_rodata = .; -/* - * .text..L.cfi.jumptable.* contain Control-Flow Integrity (CFI) - * jump table entries. - */ -#ifdef CONFIG_CFI_CLANG -#define TEXT_CFI_JT \ - . = ALIGN(PMD_SIZE); \ - __cfi_jt_start = .; \ - *(.text..L.cfi.jumptable .text..L.cfi.jumptable.*) \ - . = ALIGN(PMD_SIZE); \ - __cfi_jt_end = .; -#else -#define TEXT_CFI_JT -#endif - /* * Non-instrumentable text section */ @@ -579,7 +582,6 @@ *(.text..refcount) \ *(.ref.text) \ *(.text.asan.* .text.tsan.*) \ - TEXT_CFI_JT \ MEM_KEEP(init.text*) \ MEM_KEEP(exit.text*) \ @@ -1008,8 +1010,7 @@ * keep any .init_array.* sections. * https://bugs.llvm.org/show_bug.cgi?id=46478 */ -#if defined(CONFIG_GCOV_KERNEL) || defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KCSAN) || \ - defined(CONFIG_CFI_CLANG) +#if defined(CONFIG_GCOV_KERNEL) || defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KCSAN) # ifdef CONFIG_CONSTRUCTORS # define SANITIZER_DISCARDS \ *(.eh_frame) diff --git a/include/linux/cfi.h b/include/linux/cfi.h index 2cdbc0fbd0ab..5e134f4ce8b7 100644 --- a/include/linux/cfi.h +++ b/include/linux/cfi.h @@ -2,17 +2,38 @@ /* * Clang Control Flow Integrity (CFI) support. * - * Copyright (C) 2021 Google LLC + * Copyright (C) 2022 Google LLC */ #ifndef _LINUX_CFI_H #define _LINUX_CFI_H +#include +#include + #ifdef CONFIG_CFI_CLANG -typedef void (*cfi_check_fn)(uint64_t id, void *ptr, void *diag); +enum bug_trap_type report_cfi_failure(struct pt_regs *regs, unsigned long addr, + unsigned long *target, u32 type); -/* Compiler-generated function in each module, and the kernel */ -extern void __cfi_check(uint64_t id, void *ptr, void *diag); +static inline enum bug_trap_type report_cfi_failure_noaddr(struct pt_regs *regs, + unsigned long addr) +{ + return report_cfi_failure(regs, addr, NULL, 0); +} +#ifdef CONFIG_ARCH_USES_CFI_TRAPS +bool is_cfi_trap(unsigned long addr); +#endif #endif /* CONFIG_CFI_CLANG */ +#ifdef CONFIG_MODULES +#ifdef CONFIG_ARCH_USES_CFI_TRAPS +void module_cfi_finalize(const Elf_Ehdr *hdr, const Elf_Shdr *sechdrs, + struct module *mod); +#else +static inline void module_cfi_finalize(const Elf_Ehdr *hdr, + const Elf_Shdr *sechdrs, + struct module *mod) {} +#endif /* CONFIG_ARCH_USES_CFI_TRAPS */ +#endif /* CONFIG_MODULES */ + #endif /* _LINUX_CFI_H */ diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h index c84fec767445..42e55579d649 100644 --- a/include/linux/compiler-clang.h +++ b/include/linux/compiler-clang.h @@ -66,17 +66,9 @@ # define __noscs __attribute__((__no_sanitize__("shadow-call-stack"))) #endif -#define __nocfi __attribute__((__no_sanitize__("cfi"))) -#define __cficanonical __attribute__((__cfi_canonical_jump_table__)) - -#if defined(CONFIG_CFI_CLANG) -/* - * With CONFIG_CFI_CLANG, the compiler replaces function address - * references with the address of the function's CFI jump table - * entry. The function_nocfi macro always returns the address of the - * actual function instead. - */ -#define function_nocfi(x) __builtin_function_start(x) +#if __has_feature(kcfi) +/* Disable CFI checking inside a function. */ +#define __nocfi __attribute__((__no_sanitize__("kcfi"))) #endif /* diff --git a/include/linux/module.h b/include/linux/module.h index 8937b020ec04..ec61fb53979a 100644 --- a/include/linux/module.h +++ b/include/linux/module.h @@ -27,7 +27,6 @@ #include #include #include -#include #include #include @@ -387,8 +386,9 @@ struct module { const s32 *crcs; unsigned int num_syms; -#ifdef CONFIG_CFI_CLANG - cfi_check_fn cfi_check; +#ifdef CONFIG_ARCH_USES_CFI_TRAPS + s32 *kcfi_traps; + s32 *kcfi_traps_end; #endif /* Kernel parameters. */ diff --git a/kernel/cfi.c b/kernel/cfi.c index e8bc1b370edc..08caad776717 100644 --- a/kernel/cfi.c +++ b/kernel/cfi.c @@ -1,105 +1,101 @@ // SPDX-License-Identifier: GPL-2.0 /* - * Clang Control Flow Integrity (CFI) error and slowpath handling. + * Clang Control Flow Integrity (CFI) error handling. * - * Copyright (C) 2021 Google LLC + * Copyright (C) 2022 Google LLC */ -#include -#include -#include -#include -#include -#include -#include -#include -#include -#include +#include -/* Compiler-defined handler names */ -#ifdef CONFIG_CFI_PERMISSIVE -#define cfi_failure_handler __ubsan_handle_cfi_check_fail -#else -#define cfi_failure_handler __ubsan_handle_cfi_check_fail_abort -#endif - -static inline void handle_cfi_failure(void *ptr) +enum bug_trap_type report_cfi_failure(struct pt_regs *regs, unsigned long addr, + unsigned long *target, u32 type) { - if (IS_ENABLED(CONFIG_CFI_PERMISSIVE)) - WARN_RATELIMIT(1, "CFI failure (target: %pS):\n", ptr); + if (target) + pr_err("CFI failure at %pS (target: %pS; expected type: 0x%08x)\n", + (void *)addr, (void *)*target, type); else - panic("CFI failure (target: %pS)\n", ptr); + pr_err("CFI failure at %pS (no target information)\n", + (void *)addr); + + if (IS_ENABLED(CONFIG_CFI_PERMISSIVE)) { + __warn(NULL, 0, (void *)addr, 0, regs, NULL); + return BUG_TRAP_TYPE_WARN; + } + + return BUG_TRAP_TYPE_BUG; +} + +#ifdef CONFIG_ARCH_USES_CFI_TRAPS +static inline unsigned long trap_address(s32 *p) +{ + return (unsigned long)((long)p + (long)*p); +} + +static bool is_trap(unsigned long addr, s32 *start, s32 *end) +{ + s32 *p; + + for (p = start; p < end; ++p) { + if (trap_address(p) == addr) + return true; + } + + return false; } #ifdef CONFIG_MODULES - -static inline cfi_check_fn find_module_check_fn(unsigned long ptr) +/* Populates `kcfi_trap(_end)?` fields in `struct module`. */ +void module_cfi_finalize(const Elf_Ehdr *hdr, const Elf_Shdr *sechdrs, + struct module *mod) +{ + char *secstrings; + unsigned int i; + + mod->kcfi_traps = NULL; + mod->kcfi_traps_end = NULL; + + secstrings = (char *)hdr + sechdrs[hdr->e_shstrndx].sh_offset; + + for (i = 1; i < hdr->e_shnum; i++) { + if (strcmp(secstrings + sechdrs[i].sh_name, "__kcfi_traps")) + continue; + + mod->kcfi_traps = (s32 *)sechdrs[i].sh_addr; + mod->kcfi_traps_end = (s32 *)(sechdrs[i].sh_addr + sechdrs[i].sh_size); + break; + } +} + +static bool is_module_cfi_trap(unsigned long addr) { - cfi_check_fn fn = NULL; struct module *mod; + bool found = false; rcu_read_lock_sched_notrace(); - mod = __module_address(ptr); + + mod = __module_address(addr); if (mod) - fn = mod->cfi_check; + found = is_trap(addr, mod->kcfi_traps, mod->kcfi_traps_end); + rcu_read_unlock_sched_notrace(); - return fn; + return found; } - -static inline cfi_check_fn find_check_fn(unsigned long ptr) +#else /* CONFIG_MODULES */ +static inline bool is_module_cfi_trap(unsigned long addr) { - cfi_check_fn fn = NULL; - unsigned long flags; - bool rcu_idle; - - if (is_kernel_text(ptr)) - return __cfi_check; - - /* - * Indirect call checks can happen when RCU is not watching. Both - * the shadow and __module_address use RCU, so we need to wake it - * up if necessary. - */ - rcu_idle = !rcu_is_watching(); - if (rcu_idle) { - local_irq_save(flags); - ct_irq_enter(); - } - - fn = find_module_check_fn(ptr); - - if (rcu_idle) { - ct_irq_exit(); - local_irq_restore(flags); - } - - return fn; + return false; } - -void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag) -{ - cfi_check_fn fn = find_check_fn((unsigned long)ptr); - - if (likely(fn)) - fn(id, ptr, diag); - else /* Don't allow unchecked modules */ - handle_cfi_failure(ptr); -} -EXPORT_SYMBOL(__cfi_slowpath_diag); - -#else /* !CONFIG_MODULES */ - -void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag) -{ - handle_cfi_failure(ptr); /* No modules */ -} -EXPORT_SYMBOL(__cfi_slowpath_diag); - #endif /* CONFIG_MODULES */ -void cfi_failure_handler(void *data, void *ptr, void *vtable) +extern s32 __start___kcfi_traps[]; +extern s32 __stop___kcfi_traps[]; + +bool is_cfi_trap(unsigned long addr) { - handle_cfi_failure(ptr); + if (is_trap(addr, __start___kcfi_traps, __stop___kcfi_traps)) + return true; + + return is_module_cfi_trap(addr); } -EXPORT_SYMBOL(cfi_failure_handler); +#endif /* CONFIG_ARCH_USES_CFI_TRAPS */ diff --git a/kernel/module/main.c b/kernel/module/main.c index 0228f44b58e5..70c0b2c6fef8 100644 --- a/kernel/module/main.c +++ b/kernel/module/main.c @@ -53,6 +53,7 @@ #include #include #include +#include #include #include "internal.h" @@ -2597,8 +2598,9 @@ static int complete_formation(struct module *mod, struct load_info *info) if (err < 0) goto out; - /* This relies on module_mutex for list integrity. */ + /* These rely on module_mutex for list integrity. */ module_bug_finalize(info->hdr, info->sechdrs, mod); + module_cfi_finalize(info->hdr, info->sechdrs, mod); if (module_check_misalignment(mod)) goto out_misaligned; @@ -2660,8 +2662,6 @@ static int unknown_module_param_cb(char *param, char *val, const char *modname, return 0; } -static void cfi_init(struct module *mod); - /* * Allocate and load the module: note that size of section 0 is always * zero, and we rely on this for optional sections. @@ -2791,9 +2791,6 @@ static int load_module(struct load_info *info, const char __user *uargs, flush_module_icache(mod); - /* Setup CFI for the module. */ - cfi_init(mod); - /* Now copy in args */ mod->args = strndup_user(uargs, ~0UL >> 1); if (IS_ERR(mod->args)) { @@ -2955,32 +2952,6 @@ static inline int within(unsigned long addr, void *start, unsigned long size) return ((void *)addr >= start && (void *)addr < start + size); } -static void cfi_init(struct module *mod) -{ -#ifdef CONFIG_CFI_CLANG - initcall_t *init; -#ifdef CONFIG_MODULE_UNLOAD - exitcall_t *exit; -#endif - - rcu_read_lock_sched(); - mod->cfi_check = (cfi_check_fn) - find_kallsyms_symbol_value(mod, "__cfi_check"); - init = (initcall_t *) - find_kallsyms_symbol_value(mod, "__cfi_jt_init_module"); - /* Fix init/exit functions to point to the CFI jump table */ - if (init) - mod->init = *init; -#ifdef CONFIG_MODULE_UNLOAD - exit = (exitcall_t *) - find_kallsyms_symbol_value(mod, "__cfi_jt_cleanup_module"); - if (exit) - mod->exit = *exit; -#endif - rcu_read_unlock_sched(); -#endif -} - /* Keep in sync with MODULE_FLAGS_BUF_SIZE !!! */ char *module_flags(struct module *mod, char *buf, bool show_state) { diff --git a/scripts/module.lds.S b/scripts/module.lds.S index 3a3aa2354ed8..da4bddd26171 100644 --- a/scripts/module.lds.S +++ b/scripts/module.lds.S @@ -3,20 +3,10 @@ * Archs are free to supply their own linker scripts. ld will * combine them automatically. */ -#ifdef CONFIG_CFI_CLANG -# include -# define ALIGN_CFI ALIGN(PAGE_SIZE) -# define SANITIZER_DISCARDS *(.eh_frame) -#else -# define ALIGN_CFI -# define SANITIZER_DISCARDS -#endif - SECTIONS { /DISCARD/ : { *(.discard) *(.discard.*) - SANITIZER_DISCARDS } __ksymtab 0 : { *(SORT(___ksymtab+*)) } @@ -33,6 +23,10 @@ SECTIONS { __patchable_function_entries : { *(__patchable_function_entries) } +#ifdef CONFIG_ARCH_USES_CFI_TRAPS + __kcfi_traps : { KEEP(*(.kcfi_traps)) } +#endif + #ifdef CONFIG_LTO_CLANG /* * With CONFIG_LTO_CLANG, LLD always enables -fdata-sections and @@ -53,15 +47,6 @@ SECTIONS { *(.rodata .rodata.[0-9a-zA-Z_]*) *(.rodata..L*) } - - /* - * With CONFIG_CFI_CLANG, we assume __cfi_check is at the beginning - * of the .text section, and is aligned to PAGE_SIZE. - */ - .text : ALIGN_CFI { - *(.text.__cfi_check) - *(.text .text.[0-9a-zA-Z_]* .text..L.cfi*) - } #endif }