Re: [PATCH 2/4] fortify: Explicitly check bounds are compile-time constants

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

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@chromium.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)