[PATCH v2 31/63] fortify: Allow strlen() and strnlen() to pass compile-time known lengths

Kees Cook
2021-08-18T06:09:17Z

Under CONFIG_FORTIFY_SOURCE, it is possible for the compiler to perform
strlen() and strnlen() at compile-time when the string size is known.
This is required to support compile-time overflow checking in strlcpy().

Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/fortify-string.h | 47 ++++++++++++++++++++++++++--------
1 file changed, 36 insertions(+), 11 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index a3cb1d9aacce..e232a63fd826 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -10,6 +10,18 @@ void __read_overflow(void) __compiletime_error("detected read beyond size of obj
void __read_overflow2(void) __compiletime_error("detected read beyond size of object (2nd parameter)");
void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");

+#define __compiletime_strlen(p) ({ \
+ size_t ret = (size_t)-1; \
+ size_t p_size = __builtin_object_size(p, 1); \
+ if (p_size != (size_t)-1) { \
+ size_t p_len = p_size - 1; \
+ if (__builtin_constant_p(p[p_len]) && \
+ p[p_len] == '\0') \
+ ret = __builtin_strlen(p); \
+ } \
+ ret; \
+})
+
#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
extern void *__underlying_memchr(const void *p, int c, __kernel_size_t size) __RENAME(memchr);
extern int __underlying_memcmp(const void *p, const void *q, __kernel_size_t size) __RENAME(memcmp);
@@ -60,21 +72,31 @@ extern __kernel_size_t __real_strnlen(const char *, __kernel_size_t) __RENAME(st
__FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
{
size_t p_size = __builtin_object_size(p, 1);
- __kernel_size_t ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
+ size_t p_len = __compiletime_strlen(p);
+ size_t ret;
+
+ /* We can take compile-time actions when maxlen is const. */
+ if (__builtin_constant_p(maxlen) && p_len != (size_t)-1) {
+ /* If p is const, we can use its compile-time-known len. */
+ if (maxlen >= p_size)
+ return p_len;
+ }

+ /* Do no check characters beyond the end of p. */
+ ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
if (p_size <= ret && maxlen != ret)
fortify_panic(__func__);
return ret;
}

+/* defined after fortified strnlen to reuse it. */
__FORTIFY_INLINE __kernel_size_t strlen(const char *p)
{
__kernel_size_t ret;
size_t p_size = __builtin_object_size(p, 1);

- /* Work around gcc excess stack consumption issue */
- if (p_size == (size_t)-1 ||
- (__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
+ /* Give up if we don't know how large p is. */
+ if (p_size == (size_t)-1)
return __underlying_strlen(p);
ret = strnlen(p, p_size);
if (p_size <= ret)
@@ -86,24 +108,27 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
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)
{
- size_t ret;
size_t p_size = __builtin_object_size(p, 1);
size_t q_size = __builtin_object_size(q, 1);
+ size_t q_len; /* Full count of source string length. */
+ size_t len; /* Count of characters going into destination. */

if (p_size == (size_t)-1 && q_size == (size_t)-1)
return __real_strlcpy(p, q, size);
- ret = strlen(q);
- if (size) {
- size_t len = (ret >= size) ? size - 1 : ret;
-
- if (__builtin_constant_p(len) && len >= p_size)
+ q_len = strlen(q);
+ len = (q_len >= size) ? size - 1 : q_len;
+ if (__builtin_constant_p(size) && __builtin_constant_p(q_len) && size) {
+ /* Write size is always larger than destintation. */
+ if (len >= p_size)
__write_overflow();
+ }
+ if (size) {
if (len >= p_size)
fortify_panic(__func__);
__underlying_memcpy(p, q, len);
p[len] = '\0';
}
- return ret;
+ return q_len;
}

/* defined after fortified strnlen to reuse it */
--
2.30.2