[PATCH v6 4/4] fortify: Add Clang support

[Kees Cook <keescook@chromium.org>]

Enable FORTIFY_SOURCE support for Clang:

Use the new __pass_object_size and __overloadable attributes so
that Clang will have appropriate visibility into argument sizes such
that __builtin_object_size(p, 1) will behave correctly. Additional
details here:
    https://github.com/llvm/llvm-project/issues/53516
    https://github.com/ClangBuiltLinux/linux/issues/1401

When available, use the new __diagnose_as attribute to make sure no
compile-time diagnostic warnings are lost due to the effectively renamed
string functions.

Redefine strlen() as a macro that tests for being a constant expression
so that strlen() can still be used in static initializers, which was
lost when adding __pass_object_size and __overloadable.

Finally, a bug with __builtin_constant_p() of globally defined variables
was fixed in Clang 13 (and backported to 12.0.1), so FORTIFY support
must depend on that version or later. Additional details here:
    https://bugs.llvm.org/show_bug.cgi?id=41459
    commit a52f8a59aef4 ("fortify: Explicitly disable Clang support")

Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: George Burgess IV <gbiv@google.com>
Cc: llvm@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/fortify-string.h | 58 +++++++++++++++++++++++++---------
 security/Kconfig               |  3 +-
 2 files changed, 44 insertions(+), 17 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index c45159dbdaa1..2ffe4f2f79eb 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -2,7 +2,9 @@
 #ifndef _LINUX_FORTIFY_STRING_H_
 #define _LINUX_FORTIFY_STRING_H_
 
-#define __FORTIFY_INLINE extern __always_inline __attribute__((gnu_inline))
+#include <linux/const.h>
+
+#define __FORTIFY_INLINE extern __always_inline __attribute__((gnu_inline)) __overloadable
 #define __RENAME(x) __asm__(#x)
 
 void fortify_panic(const char *name) __noreturn __cold;
@@ -50,7 +52,17 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
 #define __underlying_strncpy	__builtin_strncpy
 #endif
 
-__FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size)
+/*
+ * Clang's use of __builtin_object_size() within inlines needs hinting via
+ * __pass_object_size(). The preference is to only ever use type 1 (member
+ * size, rather than struct size), but there remain some stragglers using
+ * type 0 that will be converted in the future.
+ */
+#define POS	__pass_object_size(1)
+#define POS0	__pass_object_size(0)
+
+__FORTIFY_INLINE __diagnose_as(__builtin_strncpy, 1, 2, 3)
+char *strncpy(char * POS p, const char *q, __kernel_size_t size)
 {
 	size_t p_size = __builtin_object_size(p, 1);
 
@@ -61,7 +73,8 @@ __FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size)
 	return __underlying_strncpy(p, q, size);
 }
 
-__FORTIFY_INLINE char *strcat(char *p, const char *q)
+__FORTIFY_INLINE __diagnose_as(__builtin_strcat, 1, 2)
+char *strcat(char * POS p, const char *q)
 {
 	size_t p_size = __builtin_object_size(p, 1);
 
@@ -73,7 +86,7 @@ __FORTIFY_INLINE char *strcat(char *p, const char *q)
 }
 
 extern __kernel_size_t __real_strnlen(const char *, __kernel_size_t) __RENAME(strnlen);
-__FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
+__FORTIFY_INLINE __kernel_size_t strnlen(const char * POS p, __kernel_size_t maxlen)
 {
 	size_t p_size = __builtin_object_size(p, 1);
 	size_t p_len = __compiletime_strlen(p);
@@ -93,8 +106,16 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
 	return ret;
 }
 
-/* defined after fortified strnlen to reuse it. */
-__FORTIFY_INLINE __kernel_size_t strlen(const char *p)
+/*
+ * Defined after fortified strnlen to reuse it. However, it must still be
+ * possible for strlen() to be used on compile-time strings for use in
+ * static initializers (i.e. as a constant expression).
+ */
+#define strlen(p)							\
+	__builtin_choose_expr(__is_constexpr(__builtin_strlen(p)),	\
+		__builtin_strlen(p), __fortify_strlen(p))
+__FORTIFY_INLINE __diagnose_as(__builtin_strlen, 1)
+__kernel_size_t __fortify_strlen(const char * POS p)
 {
 	__kernel_size_t ret;
 	size_t p_size = __builtin_object_size(p, 1);
@@ -110,7 +131,7 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
 
 /* defined after fortified strlen to reuse it */
 extern size_t __real_strlcpy(char *, const char *, size_t) __RENAME(strlcpy);
-__FORTIFY_INLINE size_t strlcpy(char *p, const char *q, size_t size)
+__FORTIFY_INLINE size_t strlcpy(char * POS p, const char * POS q, size_t size)
 {
 	size_t p_size = __builtin_object_size(p, 1);
 	size_t q_size = __builtin_object_size(q, 1);
@@ -137,7 +158,7 @@ __FORTIFY_INLINE size_t strlcpy(char *p, const char *q, size_t size)
 
 /* defined after fortified strnlen to reuse it */
 extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(strscpy);
-__FORTIFY_INLINE ssize_t strscpy(char *p, const char *q, size_t size)
+__FORTIFY_INLINE ssize_t strscpy(char * POS p, const char * POS q, size_t size)
 {
 	size_t len;
 	/* Use string size rather than possible enclosing struct size. */
@@ -183,7 +204,8 @@ __FORTIFY_INLINE ssize_t strscpy(char *p, const char *q, size_t size)
 }
 
 /* defined after fortified strlen and strnlen to reuse them */
-__FORTIFY_INLINE char *strncat(char *p, const char *q, __kernel_size_t count)
+__FORTIFY_INLINE __diagnose_as(__builtin_strncat, 1, 2, 3)
+char *strncat(char * POS p, const char * POS q, __kernel_size_t count)
 {
 	size_t p_len, copy_len;
 	size_t p_size = __builtin_object_size(p, 1);
@@ -354,7 +376,7 @@ __FORTIFY_INLINE void fortify_memcpy_chk(__kernel_size_t size,
 		memmove)
 
 extern void *__real_memscan(void *, int, __kernel_size_t) __RENAME(memscan);
-__FORTIFY_INLINE void *memscan(void *p, int c, __kernel_size_t size)
+__FORTIFY_INLINE void *memscan(void * POS0 p, int c, __kernel_size_t size)
 {
 	size_t p_size = __builtin_object_size(p, 0);
 
@@ -365,7 +387,8 @@ __FORTIFY_INLINE void *memscan(void *p, int c, __kernel_size_t size)
 	return __real_memscan(p, c, size);
 }
 
-__FORTIFY_INLINE int memcmp(const void *p, const void *q, __kernel_size_t size)
+__FORTIFY_INLINE __diagnose_as(__builtin_memcmp, 1, 2, 3)
+int memcmp(const void * POS0 p, const void * POS0 q, __kernel_size_t size)
 {
 	size_t p_size = __builtin_object_size(p, 0);
 	size_t q_size = __builtin_object_size(q, 0);
@@ -381,7 +404,8 @@ __FORTIFY_INLINE int memcmp(const void *p, const void *q, __kernel_size_t size)
 	return __underlying_memcmp(p, q, size);
 }
 
-__FORTIFY_INLINE void *memchr(const void *p, int c, __kernel_size_t size)
+__FORTIFY_INLINE __diagnose_as(__builtin_memchr, 1, 2, 3)
+void *memchr(const void * POS0 p, int c, __kernel_size_t size)
 {
 	size_t p_size = __builtin_object_size(p, 0);
 
@@ -393,7 +417,7 @@ __FORTIFY_INLINE void *memchr(const void *p, int c, __kernel_size_t size)
 }
 
 void *__real_memchr_inv(const void *s, int c, size_t n) __RENAME(memchr_inv);
-__FORTIFY_INLINE void *memchr_inv(const void *p, int c, size_t size)
+__FORTIFY_INLINE void *memchr_inv(const void * POS0 p, int c, size_t size)
 {
 	size_t p_size = __builtin_object_size(p, 0);
 
@@ -405,7 +429,7 @@ __FORTIFY_INLINE void *memchr_inv(const void *p, int c, size_t size)
 }
 
 extern void *__real_kmemdup(const void *src, size_t len, gfp_t gfp) __RENAME(kmemdup);
-__FORTIFY_INLINE void *kmemdup(const void *p, size_t size, gfp_t gfp)
+__FORTIFY_INLINE void *kmemdup(const void * POS0 p, size_t size, gfp_t gfp)
 {
 	size_t p_size = __builtin_object_size(p, 0);
 
@@ -417,7 +441,8 @@ __FORTIFY_INLINE void *kmemdup(const void *p, size_t size, gfp_t gfp)
 }
 
 /* Defined after fortified strlen to reuse it. */
-__FORTIFY_INLINE char *strcpy(char *p, const char *q)
+__FORTIFY_INLINE __diagnose_as(__builtin_strcpy, 1, 2)
+char *strcpy(char * POS p, const char * POS q)
 {
 	size_t p_size = __builtin_object_size(p, 1);
 	size_t q_size = __builtin_object_size(q, 1);
@@ -446,4 +471,7 @@ __FORTIFY_INLINE char *strcpy(char *p, const char *q)
 #undef __underlying_strncat
 #undef __underlying_strncpy
 
+#undef POS0
+#undef POS
+
 #endif /* _LINUX_FORTIFY_STRING_H_ */
diff --git a/security/Kconfig b/security/Kconfig
index 0b847f435beb..c125026ed088 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -177,9 +177,8 @@ config HARDENED_USERCOPY_PAGESPAN
 config FORTIFY_SOURCE
 	bool "Harden common str/mem functions against buffer overflows"
 	depends on ARCH_HAS_FORTIFY_SOURCE
-	# https://bugs.llvm.org/show_bug.cgi?id=50322
 	# https://bugs.llvm.org/show_bug.cgi?id=41459
-	depends on !CC_IS_CLANG
+	depends on !CC_IS_CLANG || CLANG_VERSION >= 120001
 	help
 	  Detect overflows of buffers in common string and memory functions
 	  where the compiler can determine and validate the buffer sizes.
-- 
2.30.2