[PATCH v7 3/3] overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

Kees Cook
2024-02-14T19:46:06Z

This allows replacements of the idioms "var += offset" and "var -=
offset" with the wrapping_assign_add() and wrapping_assign_sub() helpers
respectively. They will avoid wrap-around sanitizer instrumentation.

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

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: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-hardening@vger.kernel.org
---
include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++
lib/overflow_kunit.c | 43 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 75 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index d3ff8e2bec29..dede374832c9 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -81,6 +81,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
__val; \
})

+/**
+ * wrapping_assign_add() - Intentionally perform a wrapping increment assignment
+ * @var: variable to be incremented
+ * @offset: amount to add
+ *
+ * Increments @var by @offset with wrap-around. Returns the resulting
+ * value of @var. Will not trip any wrap-around sanitizers.
+ *
+ * Returns the new value of @var.
+ */
+#define wrapping_assign_add(var, offset) \
+ ({ \
+ typeof(var) *__ptr = &(var); \
+ *__ptr = wrapping_add(typeof(var), *__ptr, offset); \
+ })
+
/**
* check_sub_overflow() - Calculate subtraction with overflow checking
* @a: minuend; value to subtract from
@@ -111,6 +127,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
__val; \
})

+/**
+ * wrapping_assign_sub() - Intentionally perform a wrapping decrement assign
+ * @var: variable to be decremented
+ * @offset: amount to subtract
+ *
+ * Decrements @var by @offset with wrap-around. Returns the resulting
+ * value of @var. Will not trip any wrap-around sanitizers.
+ *
+ * Returns the new value of @var.
+ */
+#define wrapping_assign_sub(var, offset) \
+ ({ \
+ typeof(var) *__ptr = &(var); \
+ *__ptr = wrapping_sub(typeof(var), *__ptr, offset); \
+ })
+
/**
* check_mul_overflow() - Calculate multiplication with overflow checking
* @a: first factor
diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index d3fdb906d3fe..65e8a72a83bf 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -284,6 +284,45 @@ DEFINE_TEST_ARRAY(s64) = {
"Unexpected wrap " #op " macro side-effect!\n"); \
} while (0)

+static int global_counter;
+static void bump_counter(void)
+{
+ global_counter++;
+}
+
+static int get_index(void)
+{
+ volatile int index = 0;
+ bump_counter();
+ return index;
+}
+
+#define check_self_op(fmt, op, sym, a, b) do { \
+ typeof(a + 0) _a = a; \
+ typeof(b + 0) _b = b; \
+ typeof(a + 0) _a_sym = a; \
+ typeof(a + 0) _a_orig[1] = { a }; \
+ typeof(b + 0) _b_orig = b; \
+ typeof(b + 0) _b_bump = b + 1; \
+ typeof(a + 0) _r; \
+ \
+ _a_sym sym _b; \
+ _r = wrapping_ ## op(_a, _b); \
+ KUNIT_EXPECT_TRUE_MSG(test, _r == _a_sym, \
+ "expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n", \
+ a, b, _a_sym, _r); \
+ KUNIT_EXPECT_TRUE_MSG(test, _a == _a_sym, \
+ "expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n", \
+ a, b, _a_sym, _a); \
+ /* Check for internal macro side-effects. */ \
+ global_counter = 0; \
+ wrapping_ ## op(_a_orig[get_index()], _b_orig++); \
+ KUNIT_EXPECT_EQ_MSG(test, global_counter, 1, \
+ "Unexpected wrapping_" #op " macro side-effect on arg1!\n"); \
+ KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
+ "Unexpected wrapping_" #op " macro side-effect on arg2!\n"); \
+} while (0)
+
#define DEFINE_TEST_FUNC_TYPED(n, t, fmt) \
static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
{ \
@@ -293,6 +332,10 @@ static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
check_one_op(t, fmt, sub, "-", p->a, p->b, p->diff, p->d_of); \
check_one_op(t, fmt, mul, "*", p->a, p->b, p->prod, p->p_of); \
check_one_op(t, fmt, mul, "*", p->b, p->a, p->prod, p->p_of); \
+ /* wrapping_assign_{add,sub}() */ \
+ check_self_op(fmt, assign_add, +=, p->a, p->b); \
+ check_self_op(fmt, assign_add, +=, p->b, p->a); \
+ check_self_op(fmt, assign_sub, -=, p->a, p->b); \
} \
\
static void n ## _overflow_test(struct kunit *test) { \
--
2.34.1