[PATCH] userns,pidns: Verify the userns for new pid namespaces

[ebiederm@xxxxxxxxxxxx (Eric W. Biederman)]

It is pointless and confusing to allow a pid namespace hierarchy and
the user namespace hierarchy to get out of sync.  The owner of a child
pid namespace should be the owner of the parent pid namespace or
a descendant of the owner of the parent pid namespace.

Otherwise it is possible to construct scenarios where it is legal to
do something in a parent pid namespace but in a child pid namespace.

It requires use of setns into a pid namespace (but not into a user
namespace) to create such a scenario.

Add the function in_userns to help in making this determination.

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

While review a patch from Kiril Tkhai I realized we were missing this
sanity check....

 include/linux/user_namespace.h |  8 +++++++-
 kernel/pid_namespace.c         |  4 ++++
 kernel/user_namespace.c        | 18 ++++++++++++------
 3 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 32354b4b4b2b..497ed50004db 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -112,8 +112,9 @@ extern ssize_t proc_projid_map_write(struct file *, const char __user *, size_t,
 extern ssize_t proc_setgroups_write(struct file *, const char __user *, size_t, loff_t *);
 extern int proc_setgroups_show(struct seq_file *m, void *v);
 extern bool userns_may_setgroups(const struct user_namespace *ns);
+extern bool in_userns(const struct user_namespace *ancestor,
+		       const struct user_namespace *child);
 extern bool current_in_userns(const struct user_namespace *target_ns);
-
 struct ns_common *ns_get_owner(struct ns_common *ns);
 #else
 
@@ -144,6 +145,11 @@ static inline bool userns_may_setgroups(const struct user_namespace *ns)
 	return true;
 }
 
+static inline bool in_userns(const struct user_namespace *target_ns)
+{
+	return true;
+}
+
 static inline bool current_in_userns(const struct user_namespace *target_ns)
 {
 	return true;
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index de461aa0bf9a..749147f5a613 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -101,6 +101,10 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	int i;
 	int err;
 
+	err = -EINVAL;
+	if (!in_userns(parent_pid_ns->user_ns, user_ns))
+		goto out;
+
 	err = -ENOSPC;
 	if (level > MAX_PID_NS_LEVEL)
 		goto out;
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 2f735cbe05e8..7d8658fbabc8 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -986,19 +986,25 @@ bool userns_may_setgroups(const struct user_namespace *ns)
 }
 
 /*
- * Returns true if @ns is the same namespace as or a descendant of
- * @target_ns.
+ * Returns true if @child is the same namespace or a descendant of
+ * @ancestor.
  */
-bool current_in_userns(const struct user_namespace *target_ns)
+bool in_userns(const struct user_namespace *ancestor,
+	       const struct user_namespace *child)
 {
-	struct user_namespace *ns;
-	for (ns = current_user_ns(); ns; ns = ns->parent) {
-		if (ns == target_ns)
+	const struct user_namespace *ns;
+	for (ns = child; ns; ns = ns->parent) {
+		if (ns == ancestor)
 			return true;
 	}
 	return false;
 }
 
+bool current_in_userns(const struct user_namespace *target_ns)
+{
+	return in_userns(target_ns, current_user_ns());
+}
+
 static inline struct user_namespace *to_user_ns(struct ns_common *ns)
 {
 	return container_of(ns, struct user_namespace, ns);
-- 
2.10.1