Re: [PATCH v2] platform/chrome: cros_ec_dev - Fix security issue

[Olof Johansson <olof@lixom.net>]

Hi,

On Thu, Mar 03, 2016 at 11:00:13AM -0800, Gwendal Grignou wrote:
> Add a check to prevent memory scribble when sending an ioctl with .insize
> set so large that memory allocation argument overflows.
> 
> Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
> ---
>  drivers/platform/chrome/cros_ec_dev.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/platform/chrome/cros_ec_dev.c b/drivers/platform/chrome/cros_ec_dev.c
> index d45cd25..0b2f730 100644
> --- a/drivers/platform/chrome/cros_ec_dev.c
> +++ b/drivers/platform/chrome/cros_ec_dev.c
> @@ -131,13 +131,23 @@ static ssize_t ec_device_read(struct file *filp, char __user *buffer,
>  static long ec_device_ioctl_xcmd(struct cros_ec_dev *ec, void __user *arg)
>  {
>  	long ret;
> +	size_t data_size;
>  	struct cros_ec_command u_cmd;
>  	struct cros_ec_command *s_cmd;
>  
>  	if (copy_from_user(&u_cmd, arg, sizeof(u_cmd)))
>  		return -EFAULT;
>  
> -	s_cmd = kmalloc(sizeof(*s_cmd) + max(u_cmd.outsize, u_cmd.insize),
> +	/*
> +	 * Prevent malicious attack where .insize is so big that amount
> +	 * kmalloc'ed rollover, allowing memcpy to write beyond the allocated
> +	 * space.
> +	 */
> +	data_size = max(u_cmd.outsize, u_cmd.insize);
> +	if (data_size + sizeof(*s_cmd) < data_size)
> +		return -EINVAL;
> +
> +	s_cmd = kmalloc(sizeof(*s_cmd) + data_size,
>  			GFP_KERNEL);

This test does work, but it's sort of silly to even try to allow almost 4GB of
allocation here.

How about you introduce a reasonable max size for a transaction instead
(256K?), and compare data_size with that? Might want to check with the EC folks
what they expect larges transactions to be from their side, and go with
a margin above that.


Also, in your commit message you should refer to the CVE this fixes.


-Olof