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From: Nick Desaulniers <ndesaulniers@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Daniel Micay <danielmicay@gmail.com>,
	Francis Laniel <laniel_francis@privacyrequired.com>,
	Bart Van Assche <bvanassche@acm.org>,
	David Gow <davidgow@google.com>,
	linux-mm@kvack.org, clang-built-linux@googlegroups.com,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH for-next 19/25] fortify: Allow strlen() and strnlen() to pass compile-time known lengths
Date: Wed, 25 Aug 2021 15:05:56 -0700	[thread overview]
Message-ID: <CAKwvOdnrO+oagJEiBMmoHrhTJKSRwzb0DK=R_QdVjhiNzb34dg@mail.gmail.com> (raw)
In-Reply-To: <20210822075122.864511-20-keescook@chromium.org>

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
>
> --
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-- 
Thanks,
~Nick Desaulniers

  reply	other threads:[~2021-08-25 22:06 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-22  7:50 [PATCH for-next 00/25] Prepare for better FORTIFY_SOURCE Kees Cook
2021-08-22  7:50 ` [PATCH for-next 01/25] scsi: ibmvscsi: Avoid multi-field memset() overflow by aiming at srp Kees Cook
2021-08-22  7:50 ` [PATCH for-next 02/25] powerpc: Split memset() to avoid multi-field overflow Kees Cook
2021-08-22  7:51 ` [PATCH for-next 03/25] stddef: Fix kerndoc for sizeof_field() and offsetofend() Kees Cook
2021-08-22  7:51 ` [PATCH for-next 04/25] stddef: Introduce struct_group() helper macro Kees Cook
2021-08-22  7:51 ` [PATCH for-next 05/25] cxl/core: Replace unions with struct_group() Kees Cook
2021-08-22  7:51 ` [PATCH for-next 06/25] bnxt_en: Use struct_group_attr() for memcpy() region Kees Cook
2021-08-22  7:51 ` [PATCH for-next 07/25] iommu/amd: Use struct_group() " Kees Cook
2021-08-22  7:51 ` [PATCH for-next 08/25] drm/mga/mga_ioc32: " Kees Cook
2021-08-22  7:51 ` [PATCH for-next 09/25] HID: cp2112: " Kees Cook
2021-08-22  7:51 ` [PATCH for-next 10/25] HID: roccat: Use struct_group() to zero kone_mouse_event Kees Cook
2021-08-22  7:51 ` [PATCH for-next 11/25] can: flexcan: Use struct_group() to zero struct flexcan_regs regions Kees Cook
2021-08-22  7:51 ` [PATCH for-next 12/25] cm4000_cs: Use struct_group() to zero struct cm4000_dev region Kees Cook
2021-08-22  7:51 ` [PATCH for-next 13/25] compiler_types.h: Remove __compiletime_object_size() Kees Cook
2021-08-23  6:43   ` Rasmus Villemoes
2021-08-25 19:43     ` Nick Desaulniers
2021-08-22  7:51 ` [PATCH for-next 14/25] lib/string: Move helper functions out of string.c Kees Cook
2021-08-25 21:48   ` Nick Desaulniers
2021-08-26  2:47     ` Kees Cook
2021-08-26 18:08       ` Nick Desaulniers
2021-08-22  7:51 ` [PATCH for-next 15/25] fortify: Move remaining fortify helpers into fortify-string.h Kees Cook
2021-08-25 21:59   ` Nick Desaulniers
2021-08-22  7:51 ` [PATCH for-next 16/25] fortify: Explicitly disable Clang support Kees Cook
2021-08-25 19:41   ` Nick Desaulniers
2021-08-22  7:51 ` [PATCH for-next 17/25] fortify: Fix dropped strcpy() compile-time write overflow check Kees Cook
2021-08-25 21:55   ` Nick Desaulniers
2021-08-22  7:51 ` [PATCH for-next 18/25] fortify: Prepare to improve strnlen() and strlen() warnings Kees Cook
2021-08-25 22:01   ` Nick Desaulniers
2021-08-22  7:51 ` [PATCH for-next 19/25] fortify: Allow strlen() and strnlen() to pass compile-time known lengths Kees Cook
2021-08-25 22:05   ` Nick Desaulniers [this message]
2021-08-26  2:56     ` Kees Cook
2021-08-26 18:02       ` Nick Desaulniers
2021-08-22  7:51 ` [PATCH for-next 20/25] fortify: Add compile-time FORTIFY_SOURCE tests Kees Cook
2021-08-22  7:51 ` [PATCH for-next 21/25] lib: Introduce CONFIG_TEST_MEMCPY Kees Cook
2021-08-24  7:00   ` David Gow
2021-08-25  2:32     ` Kees Cook
2021-10-18 15:46   ` Arnd Bergmann
2021-10-18 19:28     ` Kees Cook
2021-08-22  7:51 ` [PATCH for-next 22/25] string.h: Introduce memset_after() for wiping trailing members/padding Kees Cook
2021-08-22  7:51 ` [PATCH for-next 23/25] xfrm: Use memset_after() to clear padding Kees Cook
2021-08-22  7:51 ` [PATCH for-next 24/25] string.h: Introduce memset_startat() for wiping trailing members and padding Kees Cook
2021-08-22  7:51 ` [PATCH for-next 25/25] btrfs: Use memset_startat() to clear end of struct Kees Cook

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