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Date: Sat, 15 Aug 2020 10:09:24 -0700
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: [PATCH v2] overflow: Add __must_check attribute to check_*() helpers
Since the destination variable of the check_*_overflow() helpers will
contain a wrapped value on failure, it would be best to make sure callers
really did check the return result of the helper. Adjust the macros to use
a bool-wrapping static inline that is marked with __must_check. This means
the macros can continue to have their type-agnostic behavior while gaining
the function attribute (that cannot be applied directly to macros).
Suggested-by: Rasmus Villemoes <linux@...musvillemoes.dk>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
v2:
- de-generalized __must_check_overflow() from being named "bool" (willy)
- fix comment typos (rasmus)
v1: https://lore.kernel.org/lkml/202008121450.405E4A3@keescook
---
include/linux/overflow.h | 39 ++++++++++++++++++++++++---------------
1 file changed, 24 insertions(+), 15 deletions(-)
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 93fcef105061..f1c4e7b56bd9 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -43,6 +43,16 @@
#define is_non_negative(a) ((a) > 0 || (a) == 0)
#define is_negative(a) (!(is_non_negative(a)))
+/*
+ * Allows for effectively applying __must_check to a macro so we can have
+ * both the type-agnostic benefits of the macros while also being able to
+ * enforce that the return value is, in fact, checked.
+ */
+static inline bool __must_check __must_check_overflow(bool overflow)
+{
+ return unlikely(overflow);
+}
+
#ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
/*
* For simplicity and code hygiene, the fallback code below insists on
@@ -52,32 +62,32 @@
* alias for __builtin_add_overflow, but add type checks similar to
* below.
*/
-#define check_add_overflow(a, b, d) ({ \
+#define check_add_overflow(a, b, d) __must_check_overflow(({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
typeof(d) __d = (d); \
(void) (&__a == &__b); \
(void) (&__a == __d); \
__builtin_add_overflow(__a, __b, __d); \
-})
+}))
-#define check_sub_overflow(a, b, d) ({ \
+#define check_sub_overflow(a, b, d) __must_check_overflow(({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
typeof(d) __d = (d); \
(void) (&__a == &__b); \
(void) (&__a == __d); \
__builtin_sub_overflow(__a, __b, __d); \
-})
+}))
-#define check_mul_overflow(a, b, d) ({ \
+#define check_mul_overflow(a, b, d) __must_check_overflow(({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
typeof(d) __d = (d); \
(void) (&__a == &__b); \
(void) (&__a == __d); \
__builtin_mul_overflow(__a, __b, __d); \
-})
+}))
#else
@@ -190,21 +200,20 @@
})
-#define check_add_overflow(a, b, d) \
+#define check_add_overflow(a, b, d) __must_check_overflow( \
__builtin_choose_expr(is_signed_type(typeof(a)), \
__signed_add_overflow(a, b, d), \
- __unsigned_add_overflow(a, b, d))
+ __unsigned_add_overflow(a, b, d)))
-#define check_sub_overflow(a, b, d) \
+#define check_sub_overflow(a, b, d) __must_check_overflow( \
__builtin_choose_expr(is_signed_type(typeof(a)), \
__signed_sub_overflow(a, b, d), \
- __unsigned_sub_overflow(a, b, d))
+ __unsigned_sub_overflow(a, b, d)))
-#define check_mul_overflow(a, b, d) \
+#define check_mul_overflow(a, b, d) __must_check_overflow( \
__builtin_choose_expr(is_signed_type(typeof(a)), \
__signed_mul_overflow(a, b, d), \
- __unsigned_mul_overflow(a, b, d))
-
+ __unsigned_mul_overflow(a, b, d)))
#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
@@ -227,7 +236,7 @@
* '*d' will hold the results of the attempted shift, but is not
* considered "safe for use" if false is returned.
*/
-#define check_shl_overflow(a, s, d) ({ \
+#define check_shl_overflow(a, s, d) __must_check_overflow(({ \
typeof(a) _a = a; \
typeof(s) _s = s; \
typeof(d) _d = d; \
@@ -237,7 +246,7 @@
*_d = (_a_full << _to_shift); \
(_to_shift != _s || is_negative(*_d) || is_negative(_a) || \
(*_d >> _to_shift) != _a); \
-})
+}))
/**
* array_size() - Calculate size of 2-dimensional array.
--
2.25.1
--
Kees Cook
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