Re: [PATCH v2 1/3] lib/vsprintf.c: Fix potential NULL deref in hex_string

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Thu, 2015-01-29 at 11:03 +0100, Rasmus Villemoes wrote:
> The helper hex_string() is broken in two ways. First, it doesn't
> increment buf regardless of whether there is room to print, so callers
> such as kasprintf() that try to probe the correct storage to allocate
> will get a too small return value. But even worse, kasprintf() (and
> likely anyone else trying to find the size of the result) pass NULL
> for buf and 0 for size, so we also have end == NULL. But this means
> that the end-1 in hex_string() is (char*)-1, so buf < end-1 is true
> and we get a NULL pointer deref. I double-checked this with a trivial
> kernel module that just did a kasprintf(GFP_KERNEL, "%14ph",
> "CrashBoomBang").
> 
> Nobody seems to be using %ph with kasprintf, but we might as well fix
> it before it hits someone.
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

> ---
>  lib/vsprintf.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index ec337f64f52d..3568e3906777 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -782,11 +782,19 @@ char *hex_string(char *buf, char *end, u8 *addr, struct printf_spec spec,
>  	if (spec.field_width > 0)
>  		len = min_t(int, spec.field_width, 64);
>  
> -	for (i = 0; i < len && buf < end - 1; i++) {
> -		buf = hex_byte_pack(buf, addr[i]);
> +	for (i = 0; i < len; ++i) {
> +		if (buf < end)
> +			*buf = hex_asc_hi(addr[i]);
> +		++buf;
> +		if (buf < end)
> +			*buf = hex_asc_lo(addr[i]);
> +		++buf;
>  
> -		if (buf < end && separator && i != len - 1)
> -			*buf++ = separator;
> +		if (separator && i != len - 1) {
> +			if (buf < end)
> +				*buf = separator;
> +			++buf;
> +		}
>  	}
>  
>  	return buf;


-- 
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy