RFC: ANDROID: add support for clang Control Flow Integrity (CFI)

This change adds the CONFIG_CFI_CLANG option, CFI error handling,
and a faster look-up table for cross module CFI checks.

Bug: 67506682
Change-Id: Ic009f0a629b552a0eb16e6d89808c7029e91447d
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
[AmitP: Rebased to newer kernels without clang LTO support]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>

Makefile

@@ -800,6 +800,30 @@ KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
 LDFLAGS_vmlinux += --gc-sections
 endif
 
+ifdef CONFIG_CFI_CLANG
+cfi-clang-flags	+= -fsanitize=cfi
+DISABLE_CFI_CLANG := -fno-sanitize=cfi
+ifdef CONFIG_MODULES
+cfi-clang-flags	+= -fsanitize-cfi-cross-dso
+DISABLE_CFI_CLANG += -fno-sanitize-cfi-cross-dso
+endif
+ifdef CONFIG_CFI_PERMISSIVE
+cfi-clang-flags	+= -fsanitize-recover=cfi -fno-sanitize-trap=cfi
+endif
+
+# allow disabling only clang CFI where needed
+export DISABLE_CFI_CLANG
+endif
+
+ifdef CONFIG_CFI
+# cfi-flags are re-tested in prepare-compiler-check
+cfi-flags	:= $(cfi-clang-flags)
+KBUILD_CFLAGS	+= $(cfi-flags)
+
+DISABLE_CFI	:= $(DISABLE_CFI_CLANG)
+export DISABLE_CFI
+endif
+
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 
@@ -1123,6 +1147,12 @@ uapi-asm-generic:
 PHONY += prepare-objtool
 prepare-objtool: $(objtool_target)
 
+ifdef cfi-flags
+  ifeq ($(call cc-option, $(cfi-flags)),)
+	@echo Cannot use CONFIG_CFI: $(cfi-flags) not supported by compiler >&2 && exit 1
+  endif
+endif
+
 # Generate some files
 # ---------------------------------------------------------------------------
 

arch/Kconfig

@@ -474,6 +474,34 @@ config STACKPROTECTOR_STRONG
 	  about 20% of all kernel functions, which increases the kernel code
 	  size by about 2%.
 
+config CFI
+	bool
+
+config CFI_PERMISSIVE
+	bool "Use CFI in permissive mode"
+	depends on CFI
+	help
+	  When selected, Control Flow Integrity (CFI) violations result in a
+	  warning instead of a kernel panic. This option is useful for finding
+	  CFI violations in drivers during development.
+
+config CFI_CLANG
+	bool "Use clang Control Flow Integrity (CFI) (EXPERIMENTAL)"
+	depends on LTO_CLANG
+	depends on KALLSYMS
+	select CFI
+	help
+	  This option enables clang Control Flow Integrity (CFI), which adds
+	  runtime checking for indirect function calls.
+
+config CFI_CLANG_SHADOW
+	bool "Use CFI shadow to speed up cross-module checks"
+	default y
+	depends on CFI_CLANG
+	help
+	  If you select this option, the kernel builds a fast look-up table of
+	  CFI check functions in loaded modules to reduce overhead.
+
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool
 	help

include/asm-generic/vmlinux.lds.h

@@ -68,6 +68,7 @@
  */
 #ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
 #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
+#define TEXT_CFI_MAIN .text.cfi .text.[0-9a-zA-Z_]*.cfi
 #define DATA_MAIN .data .data.[0-9a-zA-Z_]*
 #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
 #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
@@ -75,6 +76,7 @@
 #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
 #else
 #define TEXT_MAIN .text
+#define TEXT_CFI_MAIN .text.cfi
 #define DATA_MAIN .data
 #define SDATA_MAIN .sdata
 #define RODATA_MAIN .rodata
@@ -492,6 +494,7 @@
 		ALIGN_FUNCTION();					\
 		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
 		*(.text..refcount)					\
+		*(TEXT_CFI_MAIN) 					\
 		*(.ref.text)						\
 	MEM_KEEP(init.text*)						\
 	MEM_KEEP(exit.text*)						\

include/linux/cfi.h

@@ -0,0 +1,38 @@
+#ifndef _LINUX_CFI_H
+#define _LINUX_CFI_H
+
+#include <linux/stringify.h>
+
+#ifdef CONFIG_CFI_CLANG
+#ifdef CONFIG_MODULES
+
+typedef void (*cfi_check_fn)(uint64_t, void *, void *);
+
+/* Compiler-generated function in each module, and the kernel */
+#define CFI_CHECK_FN		__cfi_check
+#define CFI_CHECK_FN_NAME	__stringify(CFI_CHECK_FN)
+
+extern void CFI_CHECK_FN(uint64_t, void *, void *);
+
+#ifdef CONFIG_CFI_CLANG_SHADOW
+extern void cfi_module_add(struct module *mod, unsigned long min_addr,
+	unsigned long max_addr);
+
+extern void cfi_module_remove(struct module *mod, unsigned long min_addr,
+	unsigned long max_addr);
+#else
+static inline void cfi_module_add(struct module *mod, unsigned long min_addr,
+	unsigned long max_addr)
+{
+}
+
+static inline void cfi_module_remove(struct module *mod, unsigned long min_addr,
+	unsigned long max_addr)
+{
+}
+#endif /* CONFIG_CFI_CLANG_SHADOW */
+
+#endif /* CONFIG_MODULES */
+#endif /* CONFIG_CFI_CLANG */
+
+#endif /* _LINUX_CFI_H */

include/linux/compiler-clang.h

@@ -44,3 +44,7 @@
 #define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 #define __assume_aligned(a, ...)	\
 	__attribute__((__assume_aligned__(a, ## __VA_ARGS__)))
+
+#ifdef CONFIG_CFI_CLANG
+#define __nocfi		__attribute__((no_sanitize("cfi")))
+#endif

include/linux/compiler_types.h

@@ -144,6 +144,10 @@ struct ftrace_likely_data {
 #define __visible
 #endif
 
+#ifndef __nocfi
+#define __nocfi
+#endif
+
 /*
  * Assume alignment of return value.
  */

include/linux/init.h

@@ -47,7 +47,7 @@
 
 /* These are for everybody (although not all archs will actually
    discard it in modules) */
-#define __init		__section(.init.text) __cold  __latent_entropy __noinitretpoline
+#define __init		__section(.init.text) __cold  __latent_entropy __noinitretpoline __nocfi
 #define __initdata	__section(.init.data)
 #define __initconst	__section(.init.rodata)
 #define __exitdata	__section(.exit.data)

include/linux/module.h

@@ -20,6 +20,7 @@
 #include <linux/export.h>
 #include <linux/rbtree_latch.h>
 #include <linux/error-injection.h>
+#include <linux/cfi.h>
 
 #include <linux/percpu.h>
 #include <asm/module.h>
@@ -347,6 +348,10 @@ struct module {
 	const s32 *crcs;
 	unsigned int num_syms;
 
+#ifdef CONFIG_CFI_CLANG
+	cfi_check_fn cfi_check;
+#endif
+
 	/* Kernel parameters. */
 #ifdef CONFIG_SYSFS
 	struct mutex param_lock;

init/Kconfig

@@ -1972,7 +1972,7 @@ endif # MODULES
 
 config MODULES_TREE_LOOKUP
 	def_bool y
-	depends on PERF_EVENTS || TRACING
+	depends on PERF_EVENTS || TRACING || CFI_CLANG
 
 config INIT_ALL_POSSIBLE
 	bool

kernel/Makefile

@@ -34,6 +34,9 @@ KASAN_SANITIZE_kcov.o := n
 # cond_syscall is currently not LTO compatible
 CFLAGS_sys_ni.o = $(DISABLE_LTO)
 
+# Don't instrument error handlers
+CFLAGS_cfi.o = $(DISABLE_CFI_CLANG)
+
 obj-y += sched/
 obj-y += locking/
 obj-y += power/
@@ -103,6 +106,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
 obj-$(CONFIG_IRQ_WORK) += irq_work.o
 obj-$(CONFIG_CPU_PM) += cpu_pm.o
 obj-$(CONFIG_BPF) += bpf/
+obj-$(CONFIG_CFI_CLANG) += cfi.o
 
 obj-$(CONFIG_PERF_EVENTS) += events/
 

kernel/cfi.c

@@ -0,0 +1,300 @@
+/*
+ * CFI (Control Flow Integrity) error and slowpath handling
+ *
+ * Copyright (C) 2017 Google, Inc.
+ */
+
+#include <linux/gfp.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+#include <linux/ratelimit.h>
+#include <linux/rcupdate.h>
+#include <linux/spinlock.h>
+#include <asm/bug.h>
+#include <asm/cacheflush.h>
+#include <asm/memory.h>
+#include <asm/set_memory.h>
+
+/* Compiler-defined handler names */
+#ifdef CONFIG_CFI_PERMISSIVE
+#define cfi_failure_handler	__ubsan_handle_cfi_check_fail
+#define cfi_slowpath_handler	__cfi_slowpath_diag
+#else /* enforcing */
+#define cfi_failure_handler	__ubsan_handle_cfi_check_fail_abort
+#define cfi_slowpath_handler	__cfi_slowpath
+#endif /* CONFIG_CFI_PERMISSIVE */
+
+static inline void handle_cfi_failure()
+{
+#ifdef CONFIG_CFI_PERMISSIVE
+	WARN_RATELIMIT(1, "CFI failure:\n");
+#else
+	pr_err("CFI failure:\n");
+	BUG();
+#endif
+}
+
+#ifdef CONFIG_MODULES
+#ifdef CONFIG_CFI_CLANG_SHADOW
+struct shadow_range {
+	/* Module address range */
+	unsigned long mod_min_addr;
+	unsigned long mod_max_addr;
+	/* Module page range */
+	unsigned long min_page;
+	unsigned long max_page;
+};
+
+#define SHADOW_ORDER	1
+#define SHADOW_PAGES	(1 << SHADOW_ORDER)
+#define SHADOW_SIZE \
+	((SHADOW_PAGES * PAGE_SIZE - sizeof(struct shadow_range)) / sizeof(u16))
+#define SHADOW_INVALID	0xFFFF
+
+struct cfi_shadow {
+	/* Page range covered by the shadow */
+	struct shadow_range r;
+	/* Page offsets to __cfi_check functions in modules */
+	u16 shadow[SHADOW_SIZE];
+};
+
+static DEFINE_SPINLOCK(shadow_update_lock);
+static struct cfi_shadow __rcu *cfi_shadow __read_mostly = NULL;
+
+static inline int ptr_to_shadow(const struct cfi_shadow *s, unsigned long ptr)
+{
+	unsigned long index;
+	unsigned long page = ptr >> PAGE_SHIFT;
+
+	if (unlikely(page < s->r.min_page))
+		return -1; /* Outside of module area */
+
+	index = page - s->r.min_page;
+
+	if (index >= SHADOW_SIZE)
+		return -1; /* Cannot be addressed with shadow */
+
+	return (int)index;
+}
+
+static inline unsigned long shadow_to_ptr(const struct cfi_shadow *s,
+	int index)
+{
+	BUG_ON(index < 0 || index >= SHADOW_SIZE);
+
+	if (unlikely(s->shadow[index] == SHADOW_INVALID))
+		return 0;
+
+	return (s->r.min_page + s->shadow[index]) << PAGE_SHIFT;
+}
+
+static void prepare_next_shadow(const struct cfi_shadow __rcu *prev,
+		struct cfi_shadow *next)
+{
+	int i, index, check;
+
+	/* Mark everything invalid */
+	memset(next->shadow, 0xFF, sizeof(next->shadow));
+
+	if (!prev)
+		return; /* No previous shadow */
+
+	/* If the base address didn't change, update is not needed */
+	if (prev->r.min_page == next->r.min_page) {
+		memcpy(next->shadow, prev->shadow, sizeof(next->shadow));
+		return;
+	}
+
+	/* Convert the previous shadow to the new address range */
+	for (i = 0; i < SHADOW_SIZE; ++i) {
+		if (prev->shadow[i] == SHADOW_INVALID)
+			continue;
+
+		index = ptr_to_shadow(next, shadow_to_ptr(prev, i));
+		if (index < 0)
+			continue;
+
+		check = ptr_to_shadow(next,
+				shadow_to_ptr(prev, prev->shadow[i]));
+		if (check < 0)
+			continue;
+
+		next->shadow[index] = (u16)check;
+	}
+}
+
+static void add_module_to_shadow(struct cfi_shadow *s, struct module *mod)
+{
+	unsigned long ptr;
+	unsigned long min_page_addr;
+	unsigned long max_page_addr;
+	unsigned long check = (unsigned long)mod->cfi_check;
+	int check_index = ptr_to_shadow(s, check);
+
+	BUG_ON((check & PAGE_MASK) != check); /* Must be page aligned */
+
+	if (check_index < 0)
+		return; /* Module not addressable with shadow */
+
+	min_page_addr = (unsigned long)mod->core_layout.base & PAGE_MASK;
+	max_page_addr = (unsigned long)mod->core_layout.base +
+				       mod->core_layout.text_size;
+	max_page_addr &= PAGE_MASK;
+
+	/* For each page, store the check function index in the shadow */
+	for (ptr = min_page_addr; ptr <= max_page_addr; ptr += PAGE_SIZE) {
+		int index = ptr_to_shadow(s, ptr);
+		if (index >= 0) {
+			/* Assume a page only contains code for one module */
+			BUG_ON(s->shadow[index] != SHADOW_INVALID);
+			s->shadow[index] = (u16)check_index;
+		}
+	}
+}
+
+static void remove_module_from_shadow(struct cfi_shadow *s, struct module *mod)
+{
+	unsigned long ptr;
+	unsigned long min_page_addr;
+	unsigned long max_page_addr;
+
+	min_page_addr = (unsigned long)mod->core_layout.base & PAGE_MASK;
+	max_page_addr = (unsigned long)mod->core_layout.base +
+				       mod->core_layout.text_size;
+	max_page_addr &= PAGE_MASK;
+
+	for (ptr = min_page_addr; ptr <= max_page_addr; ptr += PAGE_SIZE) {
+		int index = ptr_to_shadow(s, ptr);
+		if (index >= 0)
+			s->shadow[index] = SHADOW_INVALID;
+	}
+}
+
+typedef void (*update_shadow_fn)(struct cfi_shadow *, struct module *);
+
+static void update_shadow(struct module *mod, unsigned long min_addr,
+	unsigned long max_addr, update_shadow_fn fn)
+{
+	struct cfi_shadow *prev;
+	struct cfi_shadow *next = (struct cfi_shadow *)
+		__get_free_pages(GFP_KERNEL, SHADOW_ORDER);
+
+	BUG_ON(!next);
+
+	next->r.mod_min_addr = min_addr;
+	next->r.mod_max_addr = max_addr;
+	next->r.min_page = min_addr >> PAGE_SHIFT;
+	next->r.max_page = max_addr >> PAGE_SHIFT;
+
+	spin_lock(&shadow_update_lock);
+	prev = rcu_dereference_protected(cfi_shadow, 1);
+	prepare_next_shadow(prev, next);
+
+	fn(next, mod);
+	set_memory_ro((unsigned long)next, SHADOW_PAGES);
+	rcu_assign_pointer(cfi_shadow, next);
+
+	spin_unlock(&shadow_update_lock);
+	synchronize_rcu();
+
+	if (prev) {
+		set_memory_rw((unsigned long)prev, SHADOW_PAGES);
+		free_pages((unsigned long)prev, SHADOW_ORDER);
+	}
+}
+
+void cfi_module_add(struct module *mod, unsigned long min_addr,
+	unsigned long max_addr)
+{
+	update_shadow(mod, min_addr, max_addr, add_module_to_shadow);
+}
+EXPORT_SYMBOL(cfi_module_add);
+
+void cfi_module_remove(struct module *mod, unsigned long min_addr,
+	unsigned long max_addr)
+{
+	update_shadow(mod, min_addr, max_addr, remove_module_from_shadow);
+}
+EXPORT_SYMBOL(cfi_module_remove);
+
+static inline cfi_check_fn ptr_to_check_fn(const struct cfi_shadow __rcu *s,
+	unsigned long ptr)
+{
+	int index;
+	unsigned long check;
+
+	if (unlikely(!s))
+		return NULL; /* No shadow available */
+
+	if (ptr < s->r.mod_min_addr || ptr > s->r.mod_max_addr)
+		return NULL; /* Not in a mapped module */
+
+	index = ptr_to_shadow(s, ptr);
+	if (index < 0)
+		return NULL; /* Cannot be addressed with shadow */
+
+	return (cfi_check_fn)shadow_to_ptr(s, index);
+}
+#endif /* CONFIG_CFI_CLANG_SHADOW */
+
+static inline cfi_check_fn find_module_cfi_check(void *ptr)
+{
+	struct module *mod;
+
+	preempt_disable();
+	mod = __module_address((unsigned long)ptr);
+	preempt_enable();
+
+	if (mod)
+		return mod->cfi_check;
+
+	return CFI_CHECK_FN;
+}
+
+static inline cfi_check_fn find_cfi_check(void *ptr)
+{
+#ifdef CONFIG_CFI_CLANG_SHADOW
+	cfi_check_fn f;
+
+	if (!rcu_access_pointer(cfi_shadow))
+		return CFI_CHECK_FN; /* No loaded modules */
+
+	/* Look up the __cfi_check function to use */
+	rcu_read_lock();
+	f = ptr_to_check_fn(rcu_dereference(cfi_shadow), (unsigned long)ptr);
+	rcu_read_unlock();
+
+	if (f)
+		return f;
+
+	/*
+	 * Fall back to find_module_cfi_check, which works also for a larger
+	 * module address space, but is slower.
+	 */
+#endif /* CONFIG_CFI_CLANG_SHADOW */
+
+	return find_module_cfi_check(ptr);
+}
+
+void cfi_slowpath_handler(uint64_t id, void *ptr, void *diag)
+{
+	cfi_check_fn check = find_cfi_check(ptr);
+
+	if (likely(check))
+		check(id, ptr, diag);
+	else /* Don't allow unchecked modules */
+		handle_cfi_failure();
+}
+EXPORT_SYMBOL(cfi_slowpath_handler);
+#endif /* CONFIG_MODULES */
+
+void cfi_failure_handler(void *data, void *value, void *vtable)
+{
+	handle_cfi_failure();
+}
+EXPORT_SYMBOL(cfi_failure_handler);
+
+void __cfi_check_fail(void *data, void *value)
+{
+	handle_cfi_failure();
+}

kernel/module.c

@@ -2123,6 +2123,8 @@ void __weak module_arch_freeing_init(struct module *mod)
 {
 }
 
+static void cfi_cleanup(struct module *mod);
+
 /* Free a module, remove from lists, etc. */
 static void free_module(struct module *mod)
 {
@@ -2164,6 +2166,10 @@ static void free_module(struct module *mod)
 
 	/* This may be empty, but that's OK */
 	disable_ro_nx(&mod->init_layout);
+
+	/* Clean up CFI for the module. */
+	cfi_cleanup(mod);
+
 	module_arch_freeing_init(mod);
 	module_memfree(mod->init_layout.base);
 	kfree(mod->args);
@@ -3351,6 +3357,8 @@ int __weak module_finalize(const Elf_Ehdr *hdr,
 	return 0;
 }
 
+static void cfi_init(struct module *mod);
+
 static int post_relocation(struct module *mod, const struct load_info *info)
 {
 	/* Sort exception table now relocations are done. */
@@ -3363,6 +3371,9 @@ static int post_relocation(struct module *mod, const struct load_info *info)
 	/* Setup kallsyms-specific fields. */
 	add_kallsyms(mod, info);
 
+	/* Setup CFI for the module. */
+	cfi_init(mod);
+
 	/* Arch-specific module finalizing. */
 	return module_finalize(info->hdr, info->sechdrs, mod);
 }
@@ -4132,6 +4143,22 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
 }
 #endif /* CONFIG_KALLSYMS */
 
+static void cfi_init(struct module *mod)
+{
+#ifdef CONFIG_CFI_CLANG
+	mod->cfi_check =
+		(cfi_check_fn)mod_find_symname(mod, CFI_CHECK_FN_NAME);
+	cfi_module_add(mod, module_addr_min, module_addr_max);
+#endif
+}
+
+static void cfi_cleanup(struct module *mod)
+{
+#ifdef CONFIG_CFI_CLANG
+	cfi_module_remove(mod, module_addr_min, module_addr_max);
+#endif
+}
+
 /* Maximum number of characters written by module_flags() */
 #define MODULE_FLAGS_BUF_SIZE (TAINT_FLAGS_COUNT + 4)
 

net/Kconfig

@@ -293,6 +293,7 @@ config BPF_JIT
 	bool "enable BPF Just In Time compiler"
 	depends on HAVE_CBPF_JIT || HAVE_EBPF_JIT
 	depends on MODULES
+	depends on !CFI
 	---help---
 	  Berkeley Packet Filter filtering capabilities are normally handled
 	  by an interpreter. This option allows kernel to generate a native