Re: [PATCH] scsi: 3ware: fix a missing-check bug

[adam radford <aradford@gmail.com>]

On Sat, May 5, 2018 at 10:50 PM, Wenwen Wang <wang6495@umn.edu> wrote:
> In twl_chrdev_ioctl(), the ioctl driver command is firstly copied from the
> userspace pointer 'argp' and saved to the kernel object 'driver_command'.
> Then a security check is performed on the data buffer size indicated by
> 'driver_command', which is 'driver_command.buffer_length'. If the security
> check is passed, the entire ioctl command is copied again from the 'argp'
> pointer and saved to the kernel object 'tw_ioctl'. Then, various operations
> are performed on 'tw_ioctl' according to the 'cmd'. Given that the 'argp'
> pointer resides in userspace, a malicious userspace process can race to
> change the buffer size between the two copies. This way, the user can
> bypass the security check and inject invalid data buffer size. This can
> cause potential security issues in the following execution.
>
> This patch checks the buffer size obtained in the second copy. An error
> code -EINVAL will be returned if it is not same as the original one in the
> first copy.
>
> Signed-off-by: Wenwen Wang <wang6495@umn.edu>
> ---
>  drivers/scsi/3w-sas.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/scsi/3w-sas.c b/drivers/scsi/3w-sas.c
> index cf9f2a0..ea41969 100644
> --- a/drivers/scsi/3w-sas.c
> +++ b/drivers/scsi/3w-sas.c
> @@ -757,6 +757,11 @@ static long twl_chrdev_ioctl(struct file *file, unsigned int cmd, unsigned long
>         /* Now copy down the entire ioctl */
>         if (copy_from_user(tw_ioctl, argp, driver_command.buffer_length + sizeof(TW_Ioctl_Buf_Apache) - 1))
>                 goto out3;
> +       if (tw_ioctl->driver_command.buffer_length !=
> +           driver_command.buffer_length) {
> +               retval = -EINVAL;
> +               goto out3;
> +       }
>
>         /* See which ioctl we are doing */
>         switch (cmd) {
> --
> 2.7.4
>

1. Returning -EINVAL after the copy_from_user() doesn't prevent any
invalid copy down to kernel mode from happening.
2. twl_chrdev_open() checks for capable(CAP_SYS_ADMIN):

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/3w-sas.c#n834

I don't see the point in this patch.

-Adam