[PATCH v7 2/3] overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping_mul()

Kees Cook
2024-02-14T19:46:08Z

Provide helpers that will perform wrapping addition, subtraction, or
multiplication without tripping the arithmetic wrap-around sanitizers. The
first argument is the type under which the wrap-around should happen
with. In other words, these two calls will get very different results:

wrapping_mul(int, 50, 50) == 2500
wrapping_mul(u8, 50, 50) == 196

Add to the selftests to validate behavior and lack of side-effects.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Marco Elver <elver@google.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-hardening@vger.kernel.org
---
include/linux/overflow.h | 48 ++++++++++++++++++++++++++++++++++++++++
lib/overflow_kunit.c | 24 ++++++++++++++++----
2 files changed, 68 insertions(+), 4 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index ad64d810c8aa..d3ff8e2bec29 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -65,6 +65,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
#define check_add_overflow(a, b, d) \
__must_check_overflow(__builtin_add_overflow(a, b, d))

+/**
+ * wrapping_add() - Intentionally perform a wrapping addition
+ * @type: type for result of calculation
+ * @a: first addend
+ * @b: second addend
+ *
+ * Return the potentially wrapped-around addition without
+ * tripping any wrap-around sanitizers that may be enabled.
+ */
+#define wrapping_add(type, a, b) \
+ ({ \
+ type __val; \
+ __builtin_add_overflow(a, b, &__val); \
+ __val; \
+ })
+
/**
* check_sub_overflow() - Calculate subtraction with overflow checking
* @a: minuend; value to subtract from
@@ -79,6 +95,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
#define check_sub_overflow(a, b, d) \
__must_check_overflow(__builtin_sub_overflow(a, b, d))

+/**
+ * wrapping_sub() - Intentionally perform a wrapping subtraction
+ * @type: type for result of calculation
+ * @a: minuend; value to subtract from
+ * @b: subtrahend; value to subtract from @a
+ *
+ * Return the potentially wrapped-around subtraction without
+ * tripping any wrap-around sanitizers that may be enabled.
+ */
+#define wrapping_sub(type, a, b) \
+ ({ \
+ type __val; \
+ __builtin_sub_overflow(a, b, &__val); \
+ __val; \
+ })
+
/**
* check_mul_overflow() - Calculate multiplication with overflow checking
* @a: first factor
@@ -93,6 +125,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
#define check_mul_overflow(a, b, d) \
__must_check_overflow(__builtin_mul_overflow(a, b, d))

+/**
+ * wrapping_mul() - Intentionally perform a wrapping multiplication
+ * @type: type for result of calculation
+ * @a: first factor
+ * @b: second factor
+ *
+ * Return the potentially wrapped-around multiplication without
+ * tripping any wrap-around sanitizers that may be enabled.
+ */
+#define wrapping_mul(type, a, b) \
+ ({ \
+ type __val; \
+ __builtin_mul_overflow(a, b, &__val); \
+ __val; \
+ })
+
/**
* check_shl_overflow() - Calculate a left-shifted value and check overflow
* @a: Value to be shifted
diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index c527f6b75789..d3fdb906d3fe 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -258,20 +258,36 @@ DEFINE_TEST_ARRAY(s64) = {
\
_of = check_ ## op ## _overflow(a, b, &_r); \
KUNIT_EXPECT_EQ_MSG(test, _of, of, \
- "expected "fmt" "sym" "fmt" to%s overflow (type %s)\n", \
+ "expected check "fmt" "sym" "fmt" to%s overflow (type %s)\n", \
a, b, of ? "" : " not", #t); \
KUNIT_EXPECT_EQ_MSG(test, _r, r, \
- "expected "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
+ "expected check "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
a, b, r, _r, #t); \
/* Check for internal macro side-effects. */ \
_of = check_ ## op ## _overflow(_a_orig++, _b_orig++, &_r); \
- KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, "Unexpected " #op " macro side-effect!\n"); \
- KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, "Unexpected " #op " macro side-effect!\n"); \
+ KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, \
+ "Unexpected check " #op " macro side-effect!\n"); \
+ KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
+ "Unexpected check " #op " macro side-effect!\n"); \
+ \
+ _r = wrapping_ ## op(t, a, b); \
+ KUNIT_EXPECT_TRUE_MSG(test, _r == r, \
+ "expected wrap "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
+ a, b, r, _r, #t); \
+ /* Check for internal macro side-effects. */ \
+ _a_orig = a; \
+ _b_orig = b; \
+ _r = wrapping_ ## op(t, _a_orig++, _b_orig++); \
+ KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, \
+ "Unexpected wrap " #op " macro side-effect!\n"); \
+ KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
+ "Unexpected wrap " #op " macro side-effect!\n"); \
} while (0)

#define DEFINE_TEST_FUNC_TYPED(n, t, fmt) \
static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
{ \
+ /* check_{add,sub,mul}_overflow() and wrapping_{add,sub,mul} */ \
check_one_op(t, fmt, add, "+", p->a, p->b, p->sum, p->s_of); \
check_one_op(t, fmt, add, "+", p->b, p->a, p->sum, p->s_of); \
check_one_op(t, fmt, sub, "-", p->a, p->b, p->diff, p->d_of); \
--
2.34.1