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Date:   Wed, 12 Aug 2020 14:51:52 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Leon Romanovsky <leon@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com
Subject: [PATCH] overflow: Add __must_check attribute to check_*() helpers
Message-ID: <202008121450.405E4A3@keescook>
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Sender: linux-kernel-owner@vger.kernel.org
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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@rasmusvillemoes.dk>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/overflow.h | 51 +++++++++++++++++++++++-----------------
 1 file changed, 30 insertions(+), 21 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 93fcef105061..ef7d538c2d08 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 to effectively us apply __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_bool(bool condition)
+{
+	return unlikely(condition);
+}
+
 #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_bool(({	\
 	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_bool(({	\
 	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_bool(({	\
 	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)					\
-	__builtin_choose_expr(is_signed_type(typeof(a)),		\
-			__signed_add_overflow(a, b, d),			\
-			__unsigned_add_overflow(a, b, d))
+#define check_add_overflow(a, b, d)					   \
+	__must_check_bool(__builtin_choose_expr(is_signed_type(typeof(a)), \
+				__signed_add_overflow(a, b, d),		   \
+				__unsigned_add_overflow(a, b, d)))
 
-#define check_sub_overflow(a, b, d)					\
-	__builtin_choose_expr(is_signed_type(typeof(a)),		\
-			__signed_sub_overflow(a, b, d),			\
-			__unsigned_sub_overflow(a, b, d))
-
-#define check_mul_overflow(a, b, d)					\
-	__builtin_choose_expr(is_signed_type(typeof(a)),		\
-			__signed_mul_overflow(a, b, d),			\
-			__unsigned_mul_overflow(a, b, d))
+#define check_sub_overflow(a, b, d)					   \
+	__must_check_bool(__builtin_choose_expr(is_signed_type(typeof(a)), \
+				__signed_sub_overflow(a, b, d),		   \
+				__unsigned_sub_overflow(a, b, d)))
 
+#define check_mul_overflow(a, b, d)					   \
+	__must_check_bool(__builtin_choose_expr(is_signed_type(typeof(a)), \
+				__signed_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_bool(({		\
 	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