[RFC][PATCH] Fix cap_capable to only allow owners in the parent user namespace to have caps.

[ebiederm@xmission.com (Eric W. Biederman)]

Andy Lutomirski pointed out that the current behavior of allowing the
owner of a user namespace to have all caps when that owner is not in a
parent user namespace is wrong.

This is a bug introduced by the kuid conversion which made it possible
for the owner of a user namespace to live in a child user namespace.  I
goofed and totally missed this implication.

Serge and can you please take a look and see if my corrected cap_capable
reads correctly to you.

Andy or anyone else that wants to give me a second eyeball and double
check me on this I would appreciate it.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

---

diff --git a/security/commoncap.c b/security/commoncap.c
index 6dbae46..4639f44 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -70,37 +70,44 @@ int cap_netlink_send(struct sock *sk, struct sk_buff *skb)
  *
  * NOTE WELL: cap_has_capability() cannot be used like the kernel's capable()
  * and has_capability() functions.  That is, it has the reverse semantics:
  * cap_has_capability() returns 0 when a task has a capability, but the
  * kernel's capable() and has_capability() returns 1 for this case.
  */
 int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 		int cap, int audit)
 {
 	for (;;) {
-		/* The owner of the user namespace has all caps. */
-		if (targ_ns != &init_user_ns && uid_eq(targ_ns->owner, cred->euid))
-			return 0;
+		struct user_namespace *parent_ns;
 
 		/* Do we have the necessary capabilities? */
 		if (targ_ns == cred->user_ns)
 			return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
 
 		/* Have we tried all of the parent namespaces? */
 		if (targ_ns == &init_user_ns)
 			return -EPERM;
 
+		parent_ns = targ_ns->parent;
+
+		/* 
+		 * The owner of the user namespace in the parent user
+		 * namespace has all caps.
+		 */
+		if ((parent_ns == cred->user_ns) && uid_eq(targ_ns->owner, cred->euid))
+			return 0;
+
 		/*
-		 *If you have a capability in a parent user ns, then you have
+		 * If you have a capability in a parent user ns, then you have
 		 * it over all children user namespaces as well.
 		 */
-		targ_ns = targ_ns->parent;
+		targ_ns = parent_ns;
 	}
 
 	/* We never get here */
 }
 
 /**
  * cap_settime - Determine whether the current process may set the system clock
  * @ts: The time to set
  * @tz: The timezone to set
  *