[PATCH 4/4 v5] fortify: Add Clang support

Kees Cook
2022-02-02T00:30:49Z

Enable FORTIFY_SOURCE support for Clang:

Use the new __pass_object_size and __overloadable attributes so
that Clang will have appropriate visibility into argument types 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

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, 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 | 52 ++++++++++++++++++++++++----------
security/Kconfig | 2 +-
2 files changed, 38 insertions(+), 16 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index c45159dbdaa1..5482536d3197 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 +52,11 @@ 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)
+#define BOS const __pass_object_size(1)
+#define BOS0 const __pass_object_size(0)
+
+__FORTIFY_INLINE __diagnose_as(__builtin_strncpy, 1, 2, 3)
+char *strncpy(char * BOS p, const char *q, __kernel_size_t size)
{
size_t p_size = __builtin_object_size(p, 1);

@@ -61,7 +67,8 @@ __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 * BOS p, const char *q)
{
size_t p_size = __builtin_object_size(p, 1);

@@ -73,7 +80,7 @@ __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 * BOS p, __kernel_size_t maxlen)
{
size_t p_size = __builtin_object_size(p, 1);
size_t p_len = __compiletime_strlen(p);
@@ -93,8 +100,16 @@ __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 * BOS p)
{
__kernel_size_t ret;
size_t p_size = __builtin_object_size(p, 1);
@@ -110,7 +125,7 @@ __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 * BOS p, const char * BOS q, size_t size)
{
size_t p_size = __builtin_object_size(p, 1);
size_t q_size = __builtin_object_size(q, 1);
@@ -137,7 +152,7 @@ __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 * BOS p, const char * BOS q, size_t size)
{
size_t len;
/* Use string size rather than possible enclosing struct size. */
@@ -183,7 +198,8 @@ __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 * BOS p, const char * BOS q, __kernel_size_t count)
{
size_t p_len, copy_len;
size_t p_size = __builtin_object_size(p, 1);
@@ -354,7 +370,7 @@ __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 * BOS0 p, int c, __kernel_size_t size)
{
size_t p_size = __builtin_object_size(p, 0);

@@ -365,7 +381,8 @@ __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 * BOS0 p, const void * BOS0 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 +398,8 @@ __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 * BOS0 p, int c, __kernel_size_t size)
{
size_t p_size = __builtin_object_size(p, 0);

@@ -393,7 +411,7 @@ __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 * BOS0 p, int c, size_t size)
{
size_t p_size = __builtin_object_size(p, 0);

@@ -405,7 +423,7 @@ __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 * BOS0 p, size_t size, gfp_t gfp)
{
size_t p_size = __builtin_object_size(p, 0);

@@ -417,7 +435,8 @@ __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 * BOS p, const char * BOS q)
{
size_t p_size = __builtin_object_size(p, 1);
size_t q_size = __builtin_object_size(q, 1);
@@ -446,4 +465,7 @@ __FORTIFY_INLINE char *strcpy(char *p, const char *q)
#undef __underlying_strncat
#undef __underlying_strncpy

+#undef BOS0
+#undef BOS
+
#endif /* _LINUX_FORTIFY_STRING_H_ */
diff --git a/security/Kconfig b/security/Kconfig
index 0b847f435beb..1a25a567965f 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -179,7 +179,7 @@ config FORTIFY_SOURCE
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 >= 130000
help
Detect overflows of buffers in common string and memory functions
where the compiler can determine and validate the buffer sizes.
--
2.30.2