[Devel] [PATCH] prctl: Allow local CAP_SYS_ADMIN changing exe_file

[Kirill Tkhai]

This patch is going to mainstream, Eric W.Biederman
took it to his tree.

During checkpointing and restore of userspace tasks
we bumped into the situation, that it's not possible
to restore the tasks, which user namespace does not
have uid 0 or gid 0 mapped.

People create user namespace mappings like they want,
and there is no a limitation on obligatory uid and gid
"must be mapped". So, if there is no uid 0 or gid 0
in the mapping, it's impossible to restore mm->exe_file
of the processes belonging to this user namespace.

Also, there is no a workaround. It's impossible
to create a temporary uid/gid mapping, because
only one write to /proc/[pid]/uid_map and gid_map
is allowed during a namespace lifetime.
If there is an entry, then no more mapings can't be
written. If there isn't an entry, we can't write
there too, otherwise user task won't be able
to do that in the future.

The patch changes the check, and looks for CAP_SYS_ADMIN
instead of zero uid and gid. This allows to restore
a task independently of its user namespace mappings.

Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
CC: Andrew Morton <akpm at linux-foundation.org>
CC: Serge Hallyn <serge at hallyn.com>
CC: "Eric W. Biederman" <ebiederm at xmission.com>
CC: Oleg Nesterov <oleg at redhat.com>
CC: Michal Hocko <mhocko at suse.com>
CC: Andrei Vagin <avagin at openvz.org>
CC: Cyrill Gorcunov <gorcunov at openvz.org>
CC: Stanislav Kinsburskiy <skinsbursky at virtuozzo.com>
CC: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov at openvz.org>

https://jira.sw.ru/browse/PSBM-66313

Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 kernel/sys.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index be563d93754..9a681ae8584 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2183,15 +2183,11 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
 
 	/*
 	 * Finally, make sure the caller has the rights to
-	 * change /proc/pid/exe link: only local root should
+	 * change /proc/pid/exe link: only local sys admin should
 	 * be allowed to.
 	 */
 	if (prctl_map->exe_fd != (u32)-1) {
-		struct user_namespace *ns = current_user_ns();
-		const struct cred *cred = current_cred();
-
-		if (!uid_eq(cred->uid, make_kuid(ns, 0)) ||
-		    !gid_eq(cred->gid, make_kgid(ns, 0)))
+		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
 			goto out;
 	}