[PATCH v3 3/3] overflow: Introduce inc_wrap() and dec_wrap()

Kees Cook
2024-02-05T09:12:37Z

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

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

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/overflow.h | 28 +++++++++++++++++++++++++++
lib/overflow_kunit.c | 42 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 70 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 9b8c05bdb788..6a4c18544ab1 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -82,6 +82,20 @@ static inline bool __must_check __must_check_overflow(bool overflow)
__val; \
})

+/**
+ * inc_wrap() - Intentionally perform a wrapping increment
+ * @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.
+ */
+#define inc_wrap(var, offset) \
+ ({ \
+ typeof(var) *__ptr = &(var); \
+ *__ptr = add_wrap(typeof(var), *__ptr, offset); \
+ })
+
/**
* check_sub_overflow() - Calculate subtraction with overflow checking
* @a: minuend; value to subtract from
@@ -113,6 +127,20 @@ static inline bool __must_check __must_check_overflow(bool overflow)
__val; \
})

+/**
+ * dec_wrap() - Intentionally perform a wrapping decrement
+ * @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.
+ */
+#define dec_wrap(var, offset) \
+ ({ \
+ typeof(var) *__ptr = &(var); \
+ *__ptr = sub_wrap(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 064dccd973ad..23e37bce0022 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -284,14 +284,56 @@ 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 = op ## _wrap(_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; \
+ op ## _wrap(_a_orig[get_index()], _b_orig++); \
+ KUNIT_EXPECT_EQ_MSG(test, global_counter, 1, \
+ "Unexpected " #op "_wrap() macro side-effect on arg1!\n"); \
+ KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
+ "Unexpected " #op "_wrap() 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) \
{ \
+ /* check_{add,sub,mul}_overflow() and {add,sub,mul}_wrap() */ \
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); \
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); \
+ /* {inc,dec}_wrap() */ \
+ check_self_op(fmt, inc, +=, p->a, p->b); \
+ check_self_op(fmt, inc, +=, p->b, p->a); \
+ check_self_op(fmt, dec, -=, p->a, p->b); \
} \
\
static void n ## _overflow_test(struct kunit *test) { \
--
2.34.1