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Date: Wed, 21 Sep 2022 07:48:44 -0400
From: Siddhesh Poyarekar <siddhesh@...plt.org>
To: Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org
Cc: Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Tom Rix <trix@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 2/4] fortify: Explicitly check bounds are compile-time
constants
On 2022-09-20 15:22, Kees Cook wrote:
> In preparation for replacing __builtin_object_size() with
> __builtin_dynamic_object_size(), all the compile-time size checks need
> to check that the bounds variables are, in fact, known at compile-time.
> Enforce what was guaranteed with __bos(). In other words, since all uses
> of __bos() were constant expressions, it was not required to test for
> this. When these change to __bdos(), they _may_ be constant expressions,
> and the checks are only valid when the prior condition holds. This
> results in no binary differences.
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> include/linux/fortify-string.h | 50 +++++++++++++++++++++-------------
> 1 file changed, 31 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index ff879efe94ed..71c0a432c638 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -80,6 +80,12 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
> #define POS __pass_object_size(1)
> #define POS0 __pass_object_size(0)
>
> +#define __compiletime_lessthan(bounds, length) ( \
> + __builtin_constant_p(length) && \
> + __builtin_constant_p(bounds) && \
> + bounds < length \
> +)
So with the gcc ranger, the compiler has lately been quite successful at
computing a constant `bounds < length` even though bounds and length are
not constant. So perhaps this:
#define __compiletime_lessthan (bounds, length) ( \
__builtin_constant (bounds < length) && \
bounds < length \
)
might succeed in a few more cases.
Thanks,
Sid
> +
> /**
> * strncpy - Copy a string to memory with non-guaranteed NUL padding
> *
> @@ -117,7 +123,7 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
> {
> size_t p_size = __builtin_object_size(p, 1);
>
> - if (__builtin_constant_p(size) && p_size < size)
> + if (__compiletime_lessthan(p_size, size))
> __write_overflow();
> if (p_size < size)
> fortify_panic(__func__);
> @@ -224,7 +230,7 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s
> * If size can be known at compile time and is greater than
> * p_size, generate a compile time write overflow error.
> */
> - if (__builtin_constant_p(size) && size > p_size)
> + if (__compiletime_lessthan(p_size, size))
> __write_overflow();
>
> /*
> @@ -281,15 +287,16 @@ __FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size,
> /*
> * Length argument is a constant expression, so we
> * can perform compile-time bounds checking where
> - * buffer sizes are known.
> + * buffer sizes are also known at compile time.
> */
>
> /* Error when size is larger than enclosing struct. */
> - if (p_size > p_size_field && p_size < size)
> + if (__compiletime_lessthan(p_size_field, p_size) &&
> + __compiletime_lessthan(p_size, size))
> __write_overflow();
>
> /* Warn when write size is larger than dest field. */
> - if (p_size_field < size)
> + if (__compiletime_lessthan(p_size_field, size))
> __write_overflow_field(p_size_field, size);
> }
> /*
> @@ -365,25 +372,28 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
> /*
> * Length argument is a constant expression, so we
> * can perform compile-time bounds checking where
> - * buffer sizes are known.
> + * buffer sizes are also known at compile time.
> */
>
> /* Error when size is larger than enclosing struct. */
> - if (p_size > p_size_field && p_size < size)
> + if (__compiletime_lessthan(p_size_field, p_size) &&
> + __compiletime_lessthan(p_size, size))
> __write_overflow();
> - if (q_size > q_size_field && q_size < size)
> + if (__compiletime_lessthan(q_size_field, q_size) &&
> + __compiletime_lessthan(q_size, size))
> __read_overflow2();
>
> /* Warn when write size argument larger than dest field. */
> - if (p_size_field < size)
> + if (__compiletime_lessthan(p_size_field, size))
> __write_overflow_field(p_size_field, size);
> /*
> * Warn for source field over-read when building with W=1
> * or when an over-write happened, so both can be fixed at
> * the same time.
> */
> - if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || p_size_field < size) &&
> - q_size_field < size)
> + if ((IS_ENABLED(KBUILD_EXTRA_WARN1) ||
> + __compiletime_lessthan(p_size_field, size)) &&
> + __compiletime_lessthan(q_size_field, size))
> __read_overflow2_field(q_size_field, size);
> }
> /*
> @@ -494,7 +504,7 @@ __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size)
> {
> size_t p_size = __builtin_object_size(p, 0);
>
> - if (__builtin_constant_p(size) && p_size < size)
> + if (__compiletime_lessthan(p_size, size))
> __read_overflow();
> if (p_size < size)
> fortify_panic(__func__);
> @@ -508,9 +518,9 @@ int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t
> size_t q_size = __builtin_object_size(q, 0);
>
> if (__builtin_constant_p(size)) {
> - if (p_size < size)
> + if (__compiletime_lessthan(p_size, size))
> __read_overflow();
> - if (q_size < size)
> + if (__compiletime_lessthan(q_size, size))
> __read_overflow2();
> }
> if (p_size < size || q_size < size)
> @@ -523,7 +533,7 @@ void *memchr(const void * const POS0 p, int c, __kernel_size_t size)
> {
> size_t p_size = __builtin_object_size(p, 0);
>
> - if (__builtin_constant_p(size) && p_size < size)
> + if (__compiletime_lessthan(p_size, size))
> __read_overflow();
> if (p_size < size)
> fortify_panic(__func__);
> @@ -535,7 +545,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
> {
> size_t p_size = __builtin_object_size(p, 0);
>
> - if (__builtin_constant_p(size) && p_size < size)
> + if (__compiletime_lessthan(p_size, size))
> __read_overflow();
> if (p_size < size)
> fortify_panic(__func__);
> @@ -547,7 +557,7 @@ __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp
> {
> size_t p_size = __builtin_object_size(p, 0);
>
> - if (__builtin_constant_p(size) && p_size < size)
> + if (__compiletime_lessthan(p_size, size))
> __read_overflow();
> if (p_size < size)
> fortify_panic(__func__);
> @@ -563,11 +573,13 @@ char *strcpy(char * const POS p, const char * const POS q)
> size_t size;
>
> /* If neither buffer size is known, immediately give up. */
> - if (p_size == SIZE_MAX && q_size == SIZE_MAX)
> + if (__builtin_constant_p(p_size) &&
> + __builtin_constant_p(q_size) &&
> + p_size == SIZE_MAX && q_size == SIZE_MAX)
> return __underlying_strcpy(p, q);
> size = strlen(q) + 1;
> /* Compile-time check for const size overflow. */
> - if (__builtin_constant_p(size) && p_size < size)
> + if (__compiletime_lessthan(p_size, size))
> __write_overflow();
> /* Run-time check for dynamic size overflow. */
> if (p_size < size)
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