[PATCH v3] ubsan: Reintroduce signed overflow sanitizer

Kees Cook
2024-02-05T09:37:35Z

In order to mitigate unexpected signed wrap-around[1], bring back the
signed integer overflow sanitizer. It was removed in commit 6aaa31aeb9cf
("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 annotate intentional wrap-around arithmetic, the add/sub/mul_wrap()
helpers 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_WRAP_SIGNED_target.o := n" to mark it as
wrapping, and for an entire directory, "UBSAN_WRAP_SIGNED := n" can be
used.

Additionally keep these disabled under CONFIG_COMPILE_TEST for now.

Link: https://github.com/KSPP/linux/issues/26 [1]
Cc: Justin Stitt <justinstitt@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Hao Luo <haoluo@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v3:
- split out signed overflow sanitizer so we can do each separately
v2: https://lore.kernel.org/all/20240202101311.it.893-kees@kernel.org/
v1: https://lore.kernel.org/all/20240129175033.work.813-kees@kernel.org/
---
include/linux/compiler_types.h | 9 ++++-
lib/Kconfig.ubsan | 14 +++++++
lib/test_ubsan.c | 37 ++++++++++++++++++
lib/ubsan.c | 68 ++++++++++++++++++++++++++++++++++
lib/ubsan.h | 4 ++
scripts/Makefile.lib | 3 ++
scripts/Makefile.ubsan | 3 ++
7 files changed, 137 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 6f1ca49306d2..ee9d272008a5 100644
--- a/include/linux/compiler_types.h
+++ b/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")

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 56d7653f4941..129e9bc21877 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -116,6 +116,20 @@ 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
diff --git a/lib/test_ubsan.c b/lib/test_ubsan.c
index f4ee2484d4b5..276c12140ee2 100644
--- a/lib/test_ubsan.c
+++ b/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,
diff --git a/lib/ubsan.c b/lib/ubsan.c
index df4f8d1354bb..5fc107f61934 100644
--- a/lib/ubsan.c
+++ b/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;
diff --git a/lib/ubsan.h b/lib/ubsan.h
index 5d99ab81913b..0abbbac8700d 100644
--- a/lib/ubsan.h
+++ b/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);
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 52efc520ae4f..7ce8ecccc65a 100644
--- a/scripts/Makefile.lib
+++ b/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%,, \
+ $(UBSAN_WRAP_SIGNED_$(basetarget).o)$(UBSAN_SANITIZE_$(basetarget).o)$(UBSAN_WRAP_SIGNED)$(UBSAN_SANITIZE)y), \
+ $(CFLAGS_UBSAN_WRAP_SIGNED))
endif

ifeq ($(CONFIG_KCOV),y)
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 7cf42231042b..bc957add0b4d 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -13,3 +13,6 @@ ubsan-cflags-$(CONFIG_UBSAN_ENUM) += -fsanitize=enum
ubsan-cflags-$(CONFIG_UBSAN_TRAP) += $(call cc-option,-fsanitize-trap=undefined,-fsanitize-undefined-trap-on-error)

export CFLAGS_UBSAN := $(ubsan-cflags-y)
+
+ubsan-wrap-signed-cflags-$(CONFIG_UBSAN_SIGNED_WRAP) += -fsanitize=signed-integer-overflow
+export CFLAGS_UBSAN_WRAP_SIGNED := $(ubsan-wrap-signed-cflags-y)
--
2.34.1