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 <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220908215504.3686827-6-samitolvanen@google.com

1 files changed, 70 insertions, 74 deletions

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 <linux/hardirq.h>
-#include <linux/kallsyms.h>
-#include <linux/module.h>
-#include <linux/mutex.h>
-#include <linux/printk.h>
-#include <linux/ratelimit.h>
-#include <linux/rcupdate.h>
-#include <linux/vmalloc.h>
-#include <asm/cacheflush.h>
-#include <asm/set_memory.h>
-
-/* 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)
+#include <linux/cfi.h>
+
+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_MODULES
+#ifdef CONFIG_ARCH_USES_CFI_TRAPS
+static inline unsigned long trap_address(s32 *p)
+{
+	return (unsigned long)((long)p + (long)*p);
+}
 
-static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
+static bool is_trap(unsigned long addr, s32 *start, s32 *end)
 {
-	cfi_check_fn fn = NULL;
-	struct module *mod;
+	s32 *p;
 
-	rcu_read_lock_sched_notrace();
-	mod = __module_address(ptr);
-	if (mod)
-		fn = mod->cfi_check;
-	rcu_read_unlock_sched_notrace();
+	for (p = start; p < end; ++p) {
+		if (trap_address(p) == addr)
+			return true;
+	}
 
-	return fn;
+	return false;
 }
 
-static inline cfi_check_fn find_check_fn(unsigned long ptr)
+#ifdef CONFIG_MODULES
+/* Populates `kcfi_trap(_end)?` fields in `struct module`. */
+void module_cfi_finalize(const Elf_Ehdr *hdr, const Elf_Shdr *sechdrs,
+			 struct module *mod)
 {
-	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();
-	}
+	char *secstrings;
+	unsigned int i;
 
-	fn = find_module_check_fn(ptr);
+	mod->kcfi_traps = NULL;
+	mod->kcfi_traps_end = NULL;
 
-	if (rcu_idle) {
-		ct_irq_exit();
-		local_irq_restore(flags);
-	}
+	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;
 
-	return fn;
+		mod->kcfi_traps = (s32 *)sechdrs[i].sh_addr;
+		mod->kcfi_traps_end = (s32 *)(sechdrs[i].sh_addr + sechdrs[i].sh_size);
+		break;
+	}
 }
 
-void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
+static bool is_module_cfi_trap(unsigned long addr)
 {
-	cfi_check_fn fn = find_check_fn((unsigned long)ptr);
+	struct module *mod;
+	bool found = false;
 
-	if (likely(fn))
-		fn(id, ptr, diag);
-	else /* Don't allow unchecked modules */
-		handle_cfi_failure(ptr);
-}
-EXPORT_SYMBOL(__cfi_slowpath_diag);
+	rcu_read_lock_sched_notrace();
 
-#else /* !CONFIG_MODULES */
+	mod = __module_address(addr);
+	if (mod)
+		found = is_trap(addr, mod->kcfi_traps, mod->kcfi_traps_end);
 
-void __cfi_slowpath_diag(uint64_t id, void *ptr, void *diag)
+	rcu_read_unlock_sched_notrace();
+
+	return found;
+}
+#else /* CONFIG_MODULES */
+static inline bool is_module_cfi_trap(unsigned long addr)
 {
-	handle_cfi_failure(ptr); /* No modules */
+	return false;
 }
-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 */