Re: [PATCH 2/2] selinux: add capability checks for TIOCSTI ioctl

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Jun 22, 2025 at 07:41:08PM -0600, Abhinav Saxena via B4 Relay wrote:
> From: Abhinav Saxena <xandfury@gmail.com>
> 
> The TIOCSTI ioctl currently only checks the current process's
> credentials, creating a TOCTOU vulnerability where an unprivileged
> process can open a TTY fd and pass it to a privileged process via
> SCM_RIGHTS.

If a priviliged process has a fd, what is the problem with it using this
ioctl in the firstplace?

> 
> Fix by requiring BOTH the file opener (file->f_cred) AND the current
> process to have CAP_SYS_ADMIN. This prevents privilege escalation
> while ensuring legitimate use cases continue to work.
> 
> Link: https://github.com/KSPP/linux/issues/156
> 
> Signed-off-by: Abhinav Saxena <xandfury@gmail.com>
> ---
>  security/selinux/hooks.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 595ceb314aeb..a628551873ab 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3847,6 +3847,12 @@ static int selinux_file_ioctl(struct file *file, unsigned int cmd,
>  					    CAP_OPT_NONE, true);
>  		break;
>  
> +	case TIOCSTI:
> +		if (!file_ns_capable(file, &init_user_ns, CAP_SYS_ADMIN) ||
> +		    !capable(CAP_SYS_ADMIN))
> +			error = -EPERM;
> +		break;

Are you sure this type of policy should be in the selinux core code?
Wouldn't you need a "rule" for selinux to follow (or not follow) for
this type of thing and not just a blanket change to the logic?

Also, have you looked at what userspace tools actually use this ioctl to
see if this change would break anything?

thanks,

greg k-h