[PATCH v2 2/5] userns: don't let setns() lend CAP_SETFCAP

[Josef Bacik <josef@toxicpanda.com>]

Mapping uid 0 of the parent into a user namespace requires CAP_SETFCAP,
because a task in such a namespace can write file capabilities that are
honoured in the parent.  A task that is already in the namespace has a
full capability set there and can't be asked any more, so
verify_root_map() goes by what the creator had, now ns->setfcap_level.

But setns() lets other tasks in, and they get the full set as well:

  task A: uid 0, full caps          task B: uid 0, no CAP_SETFCAP
  unshare(CLONE_NEWUSER)
                                    setns(A's user ns)
                                      allowed, euid == ns->owner
                                    write "0 0 1" to /proc/self/uid_map
                                      ns->setfcap_level is A's -> allowed

or, if A's namespace maps uid 0 already, B sets security.capability on a
file.  Either way B ends up with file capabilities that are honoured in
the initial namespace, which it could not have written there, nor in a
namespace it created itself.

Refuse setns() into a user namespace unless the joiner's CAP_SETFCAP
reaches at least as far up as the namespace's setfcap_level.  Then every
task in a namespace is entitled to what its setfcap_level says, and the
checks on the way to file capabilities can go by the namespace.  This
has to apply to every user namespace, also to those that don't map the
parent's uid 0: through an idmapped mount, a root user that only exists
in the namespace can be stored as the parent's.

The change in behaviour is that a task without CAP_SETFCAP gets -EPERM
when it tries to setns() into a user namespace created by a task that
had it, such as a root-created container.  Tasks with CAP_SETFCAP, and
unprivileged users joining their own namespaces, are not affected.

Fixes: db2e718a4798 ("capabilities: require CAP_SETFCAP to map uid 0")
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 kernel/user_namespace.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index a5a94367600d..330085a7d7d4 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -1390,6 +1390,13 @@ static int userns_install(struct nsset *nsset, struct ns_common *ns)
 	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
+	/* Everybody in the namespace gets CAP_SETFCAP that reaches as far up
+	 * as its setfcap_level, so only let in tasks whose own reaches as far.
+	 */
+	if (cap_setfcap_level(current_cred(), user_ns->parent) >
+	    user_ns->setfcap_level)
+		return -EPERM;
+
 	cred = nsset_cred(nsset);
 	if (!cred)
 		return -EINVAL;

-- 
2.55.0