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Date: Wed, 25 Aug 2021 15:05:56 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Daniel Micay <danielmicay@...il.com>,
Francis Laniel <laniel_francis@...vacyrequired.com>,
Bart Van Assche <bvanassche@....org>,
David Gow <davidgow@...gle.com>, linux-mm@...ck.org,
clang-built-linux@...glegroups.com, linux-hardening@...r.kernel.org
Subject: Re: [PATCH for-next 19/25] fortify: Allow strlen() and strnlen() to
pass compile-time known lengths
On Sun, Aug 22, 2021 at 12:57 AM Kees Cook <keescook@...omium.org> wrote:
>
> 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@...omium.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; \
> +})
Can this be a `static inline` function that accepts a `const char *`
and returns a `size_t` rather than a statement expression?
> +
> #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. */
s/no/not/
> + 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. */
s/destintation/destination/
> + 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
>
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--
Thanks,
~Nick Desaulniers
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