linux-kernel - Re: [PATCH] tty: allow TIOCSLCKTRMIOS with CAP_CHECKPOINT

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Message-ID: <20231208-begibt-besonderen-096761ac8d3b@brauner>
Date:   Fri, 8 Dec 2023 14:12:25 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Adrian Reber <areber@...hat.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH] tty: allow TIOCSLCKTRMIOS with CAP_CHECKPOINT_RESTORE

On Wed, Dec 06, 2023 at 02:43:40PM +0100, Adrian Reber wrote:
> The capability CAP_CHECKPOINT_RESTORE was introduced to allow non-root
> users to checkpoint and restore processes as non-root with CRIU.
> 
> This change extends CAP_CHECKPOINT_RESTORE to enable the CRIU option
> '--shell-job' as non-root. CRIU's man-page describes the '--shell-job'
> option like this:
> 
>   Allow one to dump shell jobs. This implies the restored task will
>   inherit session and process group ID from the criu itself. This option
>   also allows to migrate a single external tty connection, to migrate
>   applications like top.
> 
> TIOCSLCKTRMIOS can only be done if the process has CAP_SYS_ADMIN and
> this change extends it to CAP_SYS_ADMIN or CAP_CHECKPOINT_RESTORE.
> 
> With this change it is possible to checkpoint and restore processes
> which have a tty connection as non-root if CAP_CHECKPOINT_RESTORE is
> set.
> 
> Signed-off-by: Adrian Reber <areber@...hat.com>
> ---
>  drivers/tty/tty_ioctl.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
> index 4b499301a3db..95d14d7128cc 100644
> --- a/drivers/tty/tty_ioctl.c
> +++ b/drivers/tty/tty_ioctl.c
> @@ -844,7 +844,7 @@ int tty_mode_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
>  			ret = -EFAULT;
>  		return ret;
>  	case TIOCSLCKTRMIOS:
> -		if (!capable(CAP_SYS_ADMIN))
> +		if (!capable(CAP_SYS_ADMIN) && !capable(CAP_CHECKPOINT_RESTORE))

In both cases you should be able to use:

if (!checkpoint_restore_ns_capable(&init_user_ns))
	return -EPERM;

with that fixed,

Acked-by: Christian Brauner <brauner@...nel.org>

>  			return -EPERM;
>  		copy_termios_locked(real_tty, &kterm);
>  		if (user_termios_to_kernel_termios(&kterm,
> @@ -861,7 +861,7 @@ int tty_mode_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
>  			ret = -EFAULT;
>  		return ret;
>  	case TIOCSLCKTRMIOS:
> -		if (!capable(CAP_SYS_ADMIN))
> +		if (!capable(CAP_SYS_ADMIN) && !capable(CAP_CHECKPOINT_RESTORE))
>  			return -EPERM;
>  		copy_termios_locked(real_tty, &kterm);
>  		if (user_termios_to_kernel_termios_1(&kterm,
> 
> base-commit: 98b1cc82c4affc16f5598d4fa14b1858671b2263
> -- 
> 2.43.0
>