Re: [PATCH 4/8] ASoC: 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: Timur Tabi <timur@kernel.org>
> Cc: Nicolin Chen <nicoleotsuka@gmail.com>
> Cc: Xiubo Li <Xiubo.Lee@gmail.com>
> Cc: Fabio Estevam <fabio.estevam@nxp.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>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

>
> ---
>  sound/soc/fsl/imx-audmux.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/sound/soc/fsl/imx-audmux.c b/sound/soc/fsl/imx-audmux.c
> index 392d5eef356d..99e07b01a2ce 100644
> --- a/sound/soc/fsl/imx-audmux.c
> +++ b/sound/soc/fsl/imx-audmux.c
> @@ -86,49 +86,49 @@ static ssize_t audmux_read_file(struct file *file, char __user *user_buf,
>         if (!buf)
>                 return -ENOMEM;
>
> -       ret = snprintf(buf, PAGE_SIZE, "PDCR: %08x\nPTCR: %08x\n",
> +       ret = scnprintf(buf, PAGE_SIZE, "PDCR: %08x\nPTCR: %08x\n",
>                        pdcr, ptcr);
>
>         if (ptcr & IMX_AUDMUX_V2_PTCR_TFSDIR)
> -               ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +               ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                 "TxFS output from %s, ",
>                                 audmux_port_string((ptcr >> 27) & 0x7));
>         else
> -               ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +               ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                 "TxFS input, ");
>
>         if (ptcr & IMX_AUDMUX_V2_PTCR_TCLKDIR)
> -               ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +               ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                 "TxClk output from %s",
>                                 audmux_port_string((ptcr >> 22) & 0x7));
>         else
> -               ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +               ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                 "TxClk input");
>
> -       ret += snprintf(buf + ret, PAGE_SIZE - ret, "\n");
> +       ret += scnprintf(buf + ret, PAGE_SIZE - ret, "\n");
>
>         if (ptcr & IMX_AUDMUX_V2_PTCR_SYN) {
> -               ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +               ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                 "Port is symmetric");
>         } else {
>                 if (ptcr & IMX_AUDMUX_V2_PTCR_RFSDIR)
> -                       ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +                       ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                         "RxFS output from %s, ",
>                                         audmux_port_string((ptcr >> 17) & 0x7));
>                 else
> -                       ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +                       ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                         "RxFS input, ");
>
>                 if (ptcr & IMX_AUDMUX_V2_PTCR_RCLKDIR)
> -                       ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +                       ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                         "RxClk output from %s",
>                                         audmux_port_string((ptcr >> 12) & 0x7));
>                 else
> -                       ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +                       ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                                         "RxClk input");
>         }
>
> -       ret += snprintf(buf + ret, PAGE_SIZE - ret,
> +       ret += scnprintf(buf + ret, PAGE_SIZE - ret,
>                         "\nData received from %s\n",
>                         audmux_port_string((pdcr >> 13) & 0x7));
>
> --
> 2.19.2
>


-- 
Kees Cook