| Message ID | 20210822075122.864511-20-keescook@chromium.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Prepare for better FORTIFY_SOURCE | expand |
On Sun, Aug 22, 2021 at 12:57 AM Kees Cook <keescook@chromium.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@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; \ > +}) 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 > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210822075122.864511-20-keescook%40chromium.org.
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@chromium.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@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; \ > > +}) > > 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@chromium.org/ 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 > > > > -- > > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. > > 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
On Wed, Aug 25, 2021 at 7:56 PM Kees Cook <keescook@chromium.org> wrote: > > 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@chromium.org> wrote: > > > > > > 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@chromium.org/ Ah right, then consider adding a comment to encourage others not to rewrite it as such. > > Regardless, it will always collapse to a const value of either -1 or > the length of the string.
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 */
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(-)