[PATCH] kernel: allow reboots from user_ns

[Glauber Costa <glommer@parallels.com>]

Reboot support for pid namespaces is already implemented and it works
well. However, in most containers the tasks will join all namespaces,
including the now supported user namespace. In that situation, we will
fail the "capable" test and won't be able to reboot.

The proposed solution is to allow reboots for CAP_SYS_BOOT processes
in the user namespace that owns the pid namespace.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/sys.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 265b376..3b0958e 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -435,9 +435,10 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 {
 	char buffer[256];
 	int ret = 0;
+	struct pid_namespace *curr_pidns = task_active_pid_ns(current);
 
 	/* We only trust the superuser with rebooting the system. */
-	if (!capable(CAP_SYS_BOOT))
+	if (!ns_capable(curr_pidns->user_ns, CAP_SYS_BOOT))
 		return -EPERM;
 
 	/* For safety, we require "magic" arguments. */
@@ -453,7 +454,7 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 	 * pid_namespace, the command is handled by reboot_pid_ns() which will
 	 * call do_exit().
 	 */
-	ret = reboot_pid_ns(task_active_pid_ns(current), cmd);
+	ret = reboot_pid_ns(curr_pidns, cmd);
 	if (ret)
 		return ret;
 
-- 
1.8.1.2