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fortify: Allow strlen() and strnlen() to pass compile-time known lengths

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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>
This commit is contained in:
Kees Cook 2021-08-02 22:51:31 -07:00
parent 369cd2165d
commit 3009f891bb
1 changed files with 38 additions and 11 deletions
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49
include/linux/fortify-string.h
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@ -10,6 +10,20 @@ 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 __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)"); void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");
#define __compiletime_strlen(p) \
({ \
unsigned char *__p = (unsigned char *)(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) #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 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); extern int __underlying_memcmp(const void *p, const void *q, __kernel_size_t size) __RENAME(memcmp);
@ -60,21 +74,31 @@ 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) __FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
{ {
size_t p_size = __builtin_object_size(p, 1); 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 not check characters beyond the end of p. */
ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
if (p_size <= ret && maxlen != ret) if (p_size <= ret && maxlen != ret)
fortify_panic(__func__); fortify_panic(__func__);
return ret; return ret;
} }
/* defined after fortified strnlen to reuse it. */
__FORTIFY_INLINE __kernel_size_t strlen(const char *p) __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
{ {
__kernel_size_t ret; __kernel_size_t ret;
size_t p_size = __builtin_object_size(p, 1); size_t p_size = __builtin_object_size(p, 1);
/* Work around gcc excess stack consumption issue */ /* Give up if we don't know how large p is. */
if (p_size == (size_t)-1 || if (p_size == (size_t)-1)
(__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
return __underlying_strlen(p); return __underlying_strlen(p);
ret = strnlen(p, p_size); ret = strnlen(p, p_size);
if (p_size <= ret) if (p_size <= ret)
@ -86,24 +110,27 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
extern size_t __real_strlcpy(char *, const char *, size_t) __RENAME(strlcpy); 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 *p, const char *q, size_t size)
{ {
size_t ret;
size_t p_size = __builtin_object_size(p, 1); size_t p_size = __builtin_object_size(p, 1);
size_t q_size = __builtin_object_size(q, 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) if (p_size == (size_t)-1 && q_size == (size_t)-1)
return __real_strlcpy(p, q, size); return __real_strlcpy(p, q, size);
ret = strlen(q); q_len = strlen(q);
if (size) { len = (q_len >= size) ? size - 1 : q_len;
size_t len = (ret >= size) ? size - 1 : ret; if (__builtin_constant_p(size) && __builtin_constant_p(q_len) && size) {
/* Write size is always larger than destination. */
if (__builtin_constant_p(len) && len >= p_size) if (len >= p_size)
__write_overflow(); __write_overflow();
}
if (size) {
if (len >= p_size) if (len >= p_size)
fortify_panic(__func__); fortify_panic(__func__);
__underlying_memcpy(p, q, len); __underlying_memcpy(p, q, len);
p[len] = '\0'; p[len] = '\0';
} }
return ret; return q_len;
} }
/* defined after fortified strnlen to reuse it */ /* defined after fortified strnlen to reuse it */