ubsan: Reintroduce signed overflow sanitizer

In order to mitigate unexpected signed wrap-around[1], bring back the
signed integer overflow sanitizer. It was removed in commit 6aaa31ae
("ubsan: remove overflow checks") because it was effectively a no-op
when combined with -fno-strict-overflow (which correctly changes signed
overflow from being "undefined" to being explicitly "wrap around").

Compilers are adjusting their sanitizers to trap wrap-around and to
detecting common code patterns that should not be instrumented
(e.g. "var + offset < var"). Prepare for this and explicitly rename
the option from "OVERFLOW" to "WRAP" to more accurately describe the
behavior.

To annotate intentional wrap-around arithmetic, the helpers
wrapping_add/sub/mul_wrap() can be used for individual statements. At
the function level, the __signed_wrap attribute can be used to mark an
entire function as expecting its signed arithmetic to wrap around. For a
single object file the Makefile can use "UBSAN_SIGNED_WRAP_target.o := n"
to mark it as wrapping, and for an entire directory, "UBSAN_SIGNED_WRAP :=
n" can be used.

Additionally keep these disabled under CONFIG_COMPILE_TEST for now.

Link: https://github.com/KSPP/linux/issues/26 [1]
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Hao Luo <haoluo@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/compiler_types.h

@@ -282,11 +282,18 @@ struct ftrace_likely_data {
 #define __no_sanitize_or_inline __always_inline
 #endif
 
+/* Do not trap wrapping arithmetic within an annotated function. */
+#ifdef CONFIG_UBSAN_SIGNED_WRAP
+# define __signed_wrap __attribute__((no_sanitize("signed-integer-overflow")))
+#else
+# define __signed_wrap
+#endif
+
 /* Section for code which can't be instrumented at all */
 #define __noinstr_section(section)					\
 	noinline notrace __attribute((__section__(section)))		\
 	__no_kcsan __no_sanitize_address __no_profile __no_sanitize_coverage \
-	__no_sanitize_memory
+	__no_sanitize_memory __signed_wrap
 
 #define noinstr __noinstr_section(".noinstr.text")
 

lib/Kconfig.ubsan

@@ -87,7 +87,6 @@ config UBSAN_LOCAL_BOUNDS
 
 config UBSAN_SHIFT
 	bool "Perform checking for bit-shift overflows"
-	default UBSAN
 	depends on $(cc-option,-fsanitize=shift)
 	help
 	  This option enables -fsanitize=shift which checks for bit-shift
@@ -116,6 +115,20 @@ config UBSAN_UNREACHABLE
 	  This option enables -fsanitize=unreachable which checks for control
 	  flow reaching an expected-to-be-unreachable position.
 
+config UBSAN_SIGNED_WRAP
+	bool "Perform checking for signed arithmetic wrap-around"
+	default UBSAN
+	depends on !COMPILE_TEST
+	depends on $(cc-option,-fsanitize=signed-integer-overflow)
+	help
+	  This option enables -fsanitize=signed-integer-overflow which checks
+	  for wrap-around of any arithmetic operations with signed integers.
+	  This currently performs nearly no instrumentation due to the
+	  kernel's use of -fno-strict-overflow which converts all would-be
+	  arithmetic undefined behavior into wrap-around arithmetic. Future
+	  sanitizer versions will allow for wrap-around checking (rather than
+	  exclusively undefined behavior).
+
 config UBSAN_BOOL
 	bool "Perform checking for non-boolean values used as boolean"
 	default UBSAN

lib/test_ubsan.c

@@ -11,6 +11,39 @@ typedef void(*test_ubsan_fp)(void);
 			#config, IS_ENABLED(config) ? "y" : "n");	\
 	} while (0)
 
+static void test_ubsan_add_overflow(void)
+{
+	volatile int val = INT_MAX;
+
+	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
+	val += 2;
+}
+
+static void test_ubsan_sub_overflow(void)
+{
+	volatile int val = INT_MIN;
+	volatile int val2 = 2;
+
+	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
+	val -= val2;
+}
+
+static void test_ubsan_mul_overflow(void)
+{
+	volatile int val = INT_MAX / 2;
+
+	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
+	val *= 3;
+}
+
+static void test_ubsan_negate_overflow(void)
+{
+	volatile int val = INT_MIN;
+
+	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
+	val = -val;
+}
+
 static void test_ubsan_divrem_overflow(void)
 {
 	volatile int val = 16;
@@ -90,6 +123,10 @@ static void test_ubsan_misaligned_access(void)
 }
 
 static const test_ubsan_fp test_ubsan_array[] = {
+	test_ubsan_add_overflow,
+	test_ubsan_sub_overflow,
+	test_ubsan_mul_overflow,
+	test_ubsan_negate_overflow,
 	test_ubsan_shift_out_of_bounds,
 	test_ubsan_out_of_bounds,
 	test_ubsan_load_invalid_value,

lib/ubsan.c

@@ -222,6 +222,74 @@ static void ubsan_epilogue(void)
 	check_panic_on_warn("UBSAN");
 }
 
+static void handle_overflow(struct overflow_data *data, void *lhs,
+			void *rhs, char op)
+{
+
+	struct type_descriptor *type = data->type;
+	char lhs_val_str[VALUE_LENGTH];
+	char rhs_val_str[VALUE_LENGTH];
+
+	if (suppress_report(&data->location))
+		return;
+
+	ubsan_prologue(&data->location, type_is_signed(type) ?
+			"signed-integer-overflow" :
+			"unsigned-integer-overflow");
+
+	val_to_string(lhs_val_str, sizeof(lhs_val_str), type, lhs);
+	val_to_string(rhs_val_str, sizeof(rhs_val_str), type, rhs);
+	pr_err("%s %c %s cannot be represented in type %s\n",
+		lhs_val_str,
+		op,
+		rhs_val_str,
+		type->type_name);
+
+	ubsan_epilogue();
+}
+
+void __ubsan_handle_add_overflow(void *data,
+				void *lhs, void *rhs)
+{
+
+	handle_overflow(data, lhs, rhs, '+');
+}
+EXPORT_SYMBOL(__ubsan_handle_add_overflow);
+
+void __ubsan_handle_sub_overflow(void *data,
+				void *lhs, void *rhs)
+{
+	handle_overflow(data, lhs, rhs, '-');
+}
+EXPORT_SYMBOL(__ubsan_handle_sub_overflow);
+
+void __ubsan_handle_mul_overflow(void *data,
+				void *lhs, void *rhs)
+{
+	handle_overflow(data, lhs, rhs, '*');
+}
+EXPORT_SYMBOL(__ubsan_handle_mul_overflow);
+
+void __ubsan_handle_negate_overflow(void *_data, void *old_val)
+{
+	struct overflow_data *data = _data;
+	char old_val_str[VALUE_LENGTH];
+
+	if (suppress_report(&data->location))
+		return;
+
+	ubsan_prologue(&data->location, "negation-overflow");
+
+	val_to_string(old_val_str, sizeof(old_val_str), data->type, old_val);
+
+	pr_err("negation of %s cannot be represented in type %s:\n",
+		old_val_str, data->type->type_name);
+
+	ubsan_epilogue();
+}
+EXPORT_SYMBOL(__ubsan_handle_negate_overflow);
+
+
 void __ubsan_handle_divrem_overflow(void *_data, void *lhs, void *rhs)
 {
 	struct overflow_data *data = _data;

lib/ubsan.h

@@ -124,6 +124,10 @@ typedef s64 s_max;
 typedef u64 u_max;
 #endif
 
+void __ubsan_handle_add_overflow(void *data, void *lhs, void *rhs);
+void __ubsan_handle_sub_overflow(void *data, void *lhs, void *rhs);
+void __ubsan_handle_mul_overflow(void *data, void *lhs, void *rhs);
+void __ubsan_handle_negate_overflow(void *_data, void *old_val);
 void __ubsan_handle_divrem_overflow(void *_data, void *lhs, void *rhs);
 void __ubsan_handle_type_mismatch(struct type_mismatch_data *data, void *ptr);
 void __ubsan_handle_type_mismatch_v1(void *_data, void *ptr);

scripts/Makefile.lib

@@ -177,6 +177,9 @@ ifeq ($(CONFIG_UBSAN),y)
 _c_flags += $(if $(patsubst n%,, \
 		$(UBSAN_SANITIZE_$(basetarget).o)$(UBSAN_SANITIZE)y), \
 		$(CFLAGS_UBSAN))
+_c_flags += $(if $(patsubst n%,, \
+		$(UBSAN_SIGNED_WRAP_$(basetarget).o)$(UBSAN_SANITIZE_$(basetarget).o)$(UBSAN_SIGNED_WRAP)$(UBSAN_SANITIZE)y), \
+		$(CFLAGS_UBSAN_SIGNED_WRAP))
 endif
 
 ifeq ($(CONFIG_KCOV),y)

scripts/Makefile.ubsan

@@ -13,3 +13,6 @@ ubsan-cflags-$(CONFIG_UBSAN_ENUM)		+= -fsanitize=enum
 ubsan-cflags-$(CONFIG_UBSAN_TRAP)		+= $(call cc-option,-fsanitize-trap=undefined,-fsanitize-undefined-trap-on-error)
 
 export CFLAGS_UBSAN := $(ubsan-cflags-y)
+
+ubsan-signed-wrap-cflags-$(CONFIG_UBSAN_SIGNED_WRAP)     += -fsanitize=signed-integer-overflow
+export CFLAGS_UBSAN_SIGNED_WRAP := $(ubsan-signed-wrap-cflags-y)