fortify: Detect struct member overflows in memset() at compile-time

As done for memcpy(), also update memset() to use the same tightened
compile-time bounds checking under CONFIG_FORTIFY_SOURCE.

Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/fortify-string.h

@@ -175,17 +175,56 @@ __FORTIFY_INLINE char *strncat(char *p, const char *q, __kernel_size_t count)
 	return p;
 }
 
-__FORTIFY_INLINE void *memset(void *p, int c, __kernel_size_t size)
+__FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size,
+					 const size_t p_size,
+					 const size_t p_size_field)
 {
-	size_t p_size = __builtin_object_size(p, 0);
+	if (__builtin_constant_p(size)) {
+		/*
+		 * Length argument is a constant expression, so we
+		 * can perform compile-time bounds checking where
+		 * buffer sizes are known.
+		 */
 
-	if (__builtin_constant_p(size) && p_size < size)
-		__write_overflow();
-	if (p_size < size)
-		fortify_panic(__func__);
-	return __underlying_memset(p, c, size);
+		/* Error when size is larger than enclosing struct. */
+		if (p_size > p_size_field && p_size < size)
+			__write_overflow();
+
+		/* Warn when write size is larger than dest field. */
+		if (p_size_field < size)
+			__write_overflow_field();
+	}
+	/*
+	 * At this point, length argument may not be a constant expression,
+	 * so run-time bounds checking can be done where buffer sizes are
+	 * known. (This is not an "else" because the above checks may only
+	 * be compile-time warnings, and we want to still warn for run-time
+	 * overflows.)
+	 */
+
+	/*
+	 * Always stop accesses beyond the struct that contains the
+	 * field, when the buffer's remaining size is known.
+	 * (The -1 test is to optimize away checks where the buffer
+	 * lengths are unknown.)
+	 */
+	if (p_size != (size_t)(-1) && p_size < size)
+		fortify_panic("memset");
 }
 
+#define __fortify_memset_chk(p, c, size, p_size, p_size_field) ({	\
+	size_t __fortify_size = (size_t)(size);				\
+	fortify_memset_chk(__fortify_size, p_size, p_size_field),	\
+	__underlying_memset(p, c, __fortify_size);			\
+})
+
+/*
+ * __builtin_object_size() must be captured here to avoid evaluating argument
+ * side-effects further into the macro layers.
+ */
+#define memset(p, c, s) __fortify_memset_chk(p, c, s,			\
+		__builtin_object_size(p, 0), __builtin_object_size(p, 1))
+
 /*
  * To make sure the compiler can enforce protection against buffer overflows,
  * memcpy(), memmove(), and memset() must not be used beyond individual
@@ -373,7 +412,6 @@ __FORTIFY_INLINE char *strcpy(char *p, const char *q)
 /* Don't use these outside the FORITFY_SOURCE implementation */
 #undef __underlying_memchr
 #undef __underlying_memcmp
-#undef __underlying_memset
 #undef __underlying_strcat
 #undef __underlying_strcpy
 #undef __underlying_strlen

lib/test_fortify/write_overflow_field-memset.c

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#define TEST	\
+	memset(instance.buf, 0x42, sizeof(instance.buf) + 1)
+
+#include "test_fortify.h"