Re: [PATCH 6/8] ASoC: intel: skylake: change snprintf to scnprintf for possible overflow

[Kees Cook <keescook@chromium.org>]

On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@1wt.eu> wrote:
>
> From: Silvio Cesare <silvio.cesare@gmail.com>
>
> Change snprintf to scnprintf. There are generally two cases where using
> snprintf causes problems.
>
> 1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)
> In this case, if snprintf would have written more characters than what the
> buffer size (SIZE) is, then size will end up larger than SIZE. In later
> uses of snprintf, SIZE - size will result in a negative number, leading
> to problems. Note that size might already be too large by using
> size = snprintf before the code reaches a case of size += snprintf.
>
> 2) If size is ultimately used as a length parameter for a copy back to user
> space, then it will potentially allow for a buffer overflow and information
> disclosure when size is greater than SIZE. When the size is used to index
> the buffer directly, we can have memory corruption. This also means when
> size = snprintf... is used, it may also cause problems since size may become
> large.  Copying to userspace is mitigated by the HARDENED_USERCOPY kernel
> configuration.
>
> The solution to these issues is to use scnprintf which returns the number of
> characters actually written to the buffer, so the size variable will never
> exceed SIZE.
>
> Signed-off-by: Silvio Cesare <silvio.cesare@gmail.com>
> Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
> Cc: Liam Girdwood <liam.r.girdwood@linux.intel.com>
> Cc: Jie Yang <yang.jie@linux.intel.com>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Greg KH <greg@kroah.com>
> Signed-off-by: Willy Tarreau <w@1wt.eu>
>
> ---
>  sound/soc/intel/skylake/skl-debug.c | 28 ++++++++++++++--------------
>  1 file changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/sound/soc/intel/skylake/skl-debug.c b/sound/soc/intel/skylake/skl-debug.c
> index 5d7ac2ee7a3c..bb28db734fb7 100644
> --- a/sound/soc/intel/skylake/skl-debug.c
> +++ b/sound/soc/intel/skylake/skl-debug.c
> @@ -43,7 +43,7 @@ static ssize_t skl_print_pins(struct skl_module_pin *m_pin, char *buf,
>         ssize_t ret = 0;
>
>         for (i = 0; i < max_pin; i++)
> -               ret += snprintf(buf + size, MOD_BUF - size,
> +               ret += scnprintf(buf + size, MOD_BUF - size,
>                                 "%s %d\n\tModule %d\n\tInstance %d\n\t"
>                                 "In-used %s\n\tType %s\n"
>                                 "\tState %d\n\tIndex %d\n",
>

While working on a Coccinelle script to find more cases of this, I
noticed that this code is buggy: it keeps overwriting the same
position in the buf string: "buf + size" and don't take "ret" into
account at all. This needs to be:

ret += scnprintf(buf + size + ret, MOD_BUF - size - ret,

-- 
Kees Cook