linux-kernel - [PATCH v2] tty: allow TIOCSLCKTRMIOS with CAP_CHECKPOINT

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Date:   Fri,  8 Dec 2023 15:36:56 +0100
From:   Adrian Reber <areber@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Cc:     Christian Brauner <brauner@...nel.org>
Subject: [PATCH v2] tty: allow TIOCSLCKTRMIOS with CAP_CHECKPOINT_RESTORE

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.

Acked-by: Christian Brauner <brauner@...nel.org>
Signed-off-by: Adrian Reber <areber@...hat.com>
---
Changes in v2:
- replaced "if (!capable(CAP_SYS_ADMIN) && !capable(CAP_CHECKPOINT_RESTORE))"
  with "if (!checkpoint_restore_ns_capable(&init_user_ns))" as suggested
  by Christian Brauner
- v1: https://lore.kernel.org/linux-serial/20231206134340.7093-1-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..85de90eebc7b 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 (!checkpoint_restore_ns_capable(&init_user_ns))
 			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 (!checkpoint_restore_ns_capable(&init_user_ns))
 			return -EPERM;
 		copy_termios_locked(real_tty, &kterm);
 		if (user_termios_to_kernel_termios_1(&kterm,

base-commit: 98b1cc82c4affc16f5598d4fa14b1858671b2263
-- 
2.43.0