Re: [PATCH v4 8/9] vsprintf: Prevent crash when dereferencing invalid pointers

[Rasmus Villemoes]

On 2018-04-04 10:58, Petr Mladek wrote:

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3551b7957d9e..1a080a75a825 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -599,12 +599,46 @@ char *__string(char *buf, char *end, const char *s, struct printf_spec spec)
>  	return widen_string(buf, len, end, spec);
>  }
>  
> + /*
> +  * This is not a fool-proof test. 99% of the time that this will fault is
> +  * due to a bad pointer, not one that crosses into bad memory. Just test
> +  * the address to make sure it doesn't fault due to a poorly added printk
> +  * during debugging.
> +  */
> +static const char *check_pointer_access(const void *ptr)
> +{
> +	char byte;
> +
> +	if (!ptr)
> +		return "(null)";
> +
> +	if (probe_kernel_address(ptr, byte))
> +		return "(efault)";
> +
> +	return NULL;
> +}
> +
> +
> +static bool valid_pointer_access(char **buf, char *end, const void *ptr,
> +				 struct printf_spec spec)
> +{
> +	const char *err_msg;
> +
> +	err_msg = check_pointer_access(ptr);
> +	if (err_msg) {
> +		*buf = __string(*buf, end, err_msg, spec);
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
>  static noinline_for_stack
>  char *string(char *buf, char *end, const char *s,
>  		   struct printf_spec spec)
>  {
> -	if ((unsigned long)s < PAGE_SIZE)
> -		s = "(null)";
> +	if (!valid_pointer_access(&buf, end, s, spec))
> +		return buf;
>  
>  	return __string(buf, end, s, spec);
>  }

Obviously, if you do add a WARN to the check_pointer_access (and please
do), that somehow needs to be suppressed for the "%s", NULL and "%s",
ZEROPTR cases, which are grandfathered in and I think is relied upon in
some places. It should be as simple as keeping  the < PAGE_SIZE check
and do "else if (!valid...())".

Rasmus