[PATCH] tty: allow TIOCSLCKTRMIOS with CAP_CHECKPOINT_RESTORE

[PATCH] tty: allow TIOCSLCKTRMIOS with CAP_CHECKPOINT_RESTORE

[Adrian Reber]

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@redhat.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))
 			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

Re: [PATCH] tty: allow TIOCSLCKTRMIOS with CAP_CHECKPOINT_RESTORE

[Christian Brauner]

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@redhat.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@kernel.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
>