[PATCH 2/5] overflow: Expand check_add_overflow() for pointer addition

Kees Cook
2024-01-29T18:34:14Z

The check_add_overflow() helper is mostly a wrapper around
__builtin_add_overflow(), but GCC and Clang refuse to operate on pointer
arguments that would normally be allowed if the addition were open-coded.

For example, we have many places where pointer overflow is tested:

struct foo *ptr;
...
/* Check for overflow */
if (ptr + count < ptr) ...

And in order to avoid running into the overflow sanitizers in the
future, we need to rewrite these "intended" overflow checks:

if (check_add_overflow(ptr, count, &result)) ...

Frustratingly the argument type validation for __builtin_add_overflow()
is done before evaluating __builtin_choose_expr(), so for arguments to
be valid simultaneously for sizeof(*p) (when p may not be a pointer),
and __builtin_add_overflow(a, ...) (when a may be a pointer), we must
introduce wrappers that always produce a specific type (but they are
only used in the places where the bogus arguments will be ignored).

To test whether a variable is a pointer or not, introduce the __is_ptr()
helper, which uses __builtin_classify_type() to find arrays and pointers
(via the new __is_ptr_or_array() helper), and then decays arrays into
pointers (via the new __decay() helper), to distinguish pointers from
arrays.

Additionally update the unit tests to cover pointer addition.

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: llvm@lists.linux.dev
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/compiler_types.h | 10 +++++
include/linux/overflow.h | 44 ++++++++++++++++++-
lib/overflow_kunit.c | 77 ++++++++++++++++++++++++++++++----
3 files changed, 120 insertions(+), 11 deletions(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 6f1ca49306d2..d27b58fddfaa 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -375,6 +375,16 @@ struct ftrace_likely_data {
/* Are two types/vars the same type (ignoring qualifiers)? */
#define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))

+/* Is variable addressable? */
+#define __is_ptr_or_array(p) (__builtin_classify_type(p) == 5)
+
+/* Return an array decayed to a pointer. */
+#define __decay(p) \
+ (&*__builtin_choose_expr(__is_ptr_or_array(p), p, NULL))
+
+/* Report if variable is a pointer type. */
+#define __is_ptr(p) __same_type(p, __decay(p))
+
/*
* __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
* non-scalar types unchanged.
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 4e741ebb8005..210e5602e89b 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -51,6 +51,43 @@ static inline bool __must_check __must_check_overflow(bool overflow)
return unlikely(overflow);
}

+/* Always produce an integral variable expression. */
+#define __filter_integral(x) \
+ __builtin_choose_expr(!__is_ptr(x), (x), 0)
+
+/* Always produce a pointer value. */
+#define __filter_ptr(x) \
+ __builtin_choose_expr(__is_ptr(x), (x), NULL)
+
+/* Always produce a pointer to an integral value. */
+#define __filter_ptrint(x) \
+ __builtin_choose_expr(!__is_ptr(*(x)), x, &(int){ 0 })
+
+/**
+ * __check_ptr_add_overflow() - Calculate pointer addition with overflow checking
+ * @a: pointer addend
+ * @b: numeric addend
+ * @d: pointer to store sum
+ *
+ * Returns 0 on success, 1 on wrap-around.
+ *
+ * Do not use this function directly, use check_add_overflow() instead.
+ *
+ * *@d holds the results of the attempted addition, which may wrap-around.
+ */
+#define __check_ptr_add_overflow(a, b, d) \
+ ({ \
+ typeof(a) __a = (a); \
+ typeof(b) __b = (b); \
+ size_t __bytes; \
+ bool __overflow; \
+ \
+ /* we want to perform the wrap-around, but retain the result */ \
+ __overflow = __builtin_mul_overflow(sizeof(*(__a)), __b, &__bytes); \
+ __builtin_add_overflow((unsigned long)(__a), __bytes, (unsigned long *)(d)) || \
+ __overflow; \
+ })
+
/**
* check_add_overflow() - Calculate addition with overflow checking
* @a: first addend
@@ -61,8 +98,11 @@ static inline bool __must_check __must_check_overflow(bool overflow)
*
* *@d holds the results of the attempted addition, which may wrap-around.
*/
-#define check_add_overflow(a, b, d) \
- __must_check_overflow(__builtin_add_overflow(a, b, d))
+#define check_add_overflow(a, b, d) \
+ __must_check_overflow(__builtin_choose_expr(__is_ptr(a), \
+ __check_ptr_add_overflow(__filter_ptr(a), b, d), \
+ __builtin_add_overflow(__filter_integral(a), b, \
+ __filter_ptrint(d))))

/**
* check_sub_overflow() - Calculate subtraction with overflow checking
diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index c527f6b75789..2d106e880956 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -45,13 +45,18 @@
# define SKIP_64_ON_32(t) do { } while (0)
#endif

-#define DEFINE_TEST_ARRAY_TYPED(t1, t2, t) \
- static const struct test_ ## t1 ## _ ## t2 ## __ ## t { \
+#define DEFINE_TEST_ARRAY_NAMED_TYPED(n1, n2, n, t1, t2, t) \
+ static const struct test_ ## n1 ## _ ## n2 ## __ ## n { \
t1 a; \
t2 b; \
- t sum, diff, prod; \
+ t sum; \
+ t diff; \
+ t prod; \
bool s_of, d_of, p_of; \
- } t1 ## _ ## t2 ## __ ## t ## _tests[]
+ } n1 ## _ ## n2 ## __ ## n ## _tests[]
+
+#define DEFINE_TEST_ARRAY_TYPED(t1, t2, t) \
+ DEFINE_TEST_ARRAY_NAMED_TYPED(t1, t2, t, t1, t2, t)

#define DEFINE_TEST_ARRAY(t) DEFINE_TEST_ARRAY_TYPED(t, t, t)

@@ -251,8 +256,10 @@ DEFINE_TEST_ARRAY(s64) = {
};

#define check_one_op(t, fmt, op, sym, a, b, r, of) do { \
- int _a_orig = a, _a_bump = a + 1; \
- int _b_orig = b, _b_bump = b + 1; \
+ typeof(a + 0) _a_orig = a; \
+ typeof(a + 0) _a_bump = a + 1; \
+ typeof(b + 0) _b_orig = b; \
+ typeof(b + 0) _b_bump = b + 1; \
bool _of; \
t _r; \
\
@@ -260,13 +267,13 @@ DEFINE_TEST_ARRAY(s64) = {
KUNIT_EXPECT_EQ_MSG(test, _of, of, \
"expected "fmt" "sym" "fmt" to%s overflow (type %s)\n", \
a, b, of ? "" : " not", #t); \
- KUNIT_EXPECT_EQ_MSG(test, _r, r, \
+ KUNIT_EXPECT_TRUE_MSG(test, _r == r, \
"expected "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_TRUE_MSG(test, _a_orig == _a_bump, "Unexpected " #op " macro side-effect!\n"); \
+ KUNIT_EXPECT_TRUE_MSG(test, _b_orig == _b_bump, "Unexpected " #op " macro side-effect!\n"); \
} while (0)

#define DEFINE_TEST_FUNC_TYPED(n, t, fmt) \
@@ -333,6 +340,55 @@ DEFINE_TEST_ARRAY_TYPED(int, int, u8) = {
};
DEFINE_TEST_FUNC_TYPED(int_int__u8, u8, "%d");

+#define DEFINE_TEST_PTR_FUNC_TYPED(n, t, fmt) \
+static void do_ptr_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
+{ \
+ /* we're only doing single-direction sums, no product or division */ \
+ check_one_op(t, fmt, add, "+", p->a, p->b, p->sum, p->s_of);\
+} \
+ \
+static void n ## _overflow_test(struct kunit *test) { \
+ unsigned i; \
+ \
+ for (i = 0; i < ARRAY_SIZE(n ## _tests); ++i) \
+ do_ptr_test_ ## n(test, &n ## _tests[i]); \
+ kunit_info(test, "%zu %s arithmetic tests finished\n", \
+ ARRAY_SIZE(n ## _tests), #n); \
+}
+
+DEFINE_TEST_ARRAY_NAMED_TYPED(void, int, void, void *, int, void *) = {
+ {NULL, 0, NULL, NULL, NULL, false, false, false},
+ {(void *)0x30, 0x10, (void *)0x40, NULL, NULL, false, false, false},
+ {(void *)ULONG_MAX, 0, (void *)ULONG_MAX, NULL, NULL, false, false, false},
+ {(void *)ULONG_MAX, 1, NULL, NULL, NULL, true, false, false},
+ {(void *)ULONG_MAX, INT_MAX, (void *)(INT_MAX - 1), NULL, NULL, true, false, false},
+};
+DEFINE_TEST_PTR_FUNC_TYPED(void_int__void, void *, "%lx");
+
+struct _sized {
+ int a;
+ char b;
+};
+
+DEFINE_TEST_ARRAY_NAMED_TYPED(sized, int, sized, struct _sized *, int, struct _sized *) = {
+ {NULL, 0, NULL, NULL, NULL, false, false, false},
+ {NULL, 1, (struct _sized *)(sizeof(struct _sized)), NULL, NULL, false, false, false},
+ {NULL, 0x10, (struct _sized *)(sizeof(struct _sized) * 0x10), NULL, NULL, false, false, false},
+ {(void *)(ULONG_MAX - sizeof(struct _sized)), 1, (struct _sized *)ULONG_MAX, NULL, NULL, false, false, false},
+ {(void *)(ULONG_MAX - sizeof(struct _sized) + 1), 1, NULL, NULL, NULL, true, false, false},
+ {(void *)(ULONG_MAX - sizeof(struct _sized) + 1), 2, (struct _sized *)(sizeof(struct _sized)), NULL, NULL, true, false, false},
+ {(void *)(ULONG_MAX - sizeof(struct _sized) + 1), 3, (struct _sized *)(sizeof(struct _sized) * 2), NULL, NULL, true, false, false},
+};
+DEFINE_TEST_PTR_FUNC_TYPED(sized_int__sized, struct _sized *, "%lx");
+
+DEFINE_TEST_ARRAY_NAMED_TYPED(sized, size_t, sized, struct _sized *, size_t, struct _sized *) = {
+ {NULL, 0, NULL, NULL, NULL, false, false, false},
+ {NULL, 1, (struct _sized *)(sizeof(struct _sized)), NULL, NULL, false, false, false},
+ {NULL, 0x10, (struct _sized *)(sizeof(struct _sized) * 0x10), NULL, NULL, false, false, false},
+ {NULL, SIZE_MAX - 10, (struct _sized *)18446744073709551528UL, NULL, NULL, true, false, false},
+};
+DEFINE_TEST_PTR_FUNC_TYPED(sized_size_t__sized, struct _sized *, "%zu");
+
/* Args are: value, shift, type, expected result, overflow expected */
#define TEST_ONE_SHIFT(a, s, t, expect, of) do { \
typeof(a) __a = (a); \
@@ -1122,6 +1178,9 @@ static struct kunit_case overflow_test_cases[] = {
KUNIT_CASE(s32_s32__s32_overflow_test),
KUNIT_CASE(u64_u64__u64_overflow_test),
KUNIT_CASE(s64_s64__s64_overflow_test),
+ KUNIT_CASE(void_int__void_overflow_test),
+ KUNIT_CASE(sized_int__sized_overflow_test),
+ KUNIT_CASE(sized_size_t__sized_overflow_test),
KUNIT_CASE(u32_u32__int_overflow_test),
KUNIT_CASE(u32_u32__u8_overflow_test),
KUNIT_CASE(u8_u8__int_overflow_test),
--
2.34.1