Re: [PATCH for-next 19/25] fortify: Allow strlen() and strnlen() to pass compile-time known lengths
From: Kees Cook
Date: Wed Aug 25 2021 - 22:56:42 EST
On Wed, Aug 25, 2021 at 03:05:56PM -0700, Nick Desaulniers wrote:
> On Sun, Aug 22, 2021 at 12:57 AM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > ---
> > 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?
No because both __builtin_object_size() and __builtin_strlen() may not
work. See:
https://lore.kernel.org/lkml/20210818060533.3569517-64-keescook@xxxxxxxxxxxx/
Regardless, it will always collapse to a const value of either -1 or
the length of the string.
>
> > +
> > #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/
Thanks!
>
> > + 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/
I can't type. :)
Fixed now.
-Kees
>
> > + 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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> > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210822075122.864511-20-keescook%40chromium.org.
>
>
>
> --
> Thanks,
> ~Nick Desaulniers
--
Kees Cook