[155/251] userns: limit the maximum depth of user_namespace->parent chain

[Steven Rostedt]

3.6.11.9-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@xxxxxxxxxx>

[ Upstream commit 8742f229b635bf1c1c84a3dfe5e47c814c20b5c8 ]

Ensure that user_namespace->parent chain can't grow too much.
Currently we use the hardroded 32 as limit.

Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 include/linux/user_namespace.h |    1 +
 kernel/user_namespace.c        |    4 ++++
 2 files changed, 5 insertions(+)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 4e72922..4eea4ce 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -22,6 +22,7 @@ struct user_namespace {
 	struct uid_gid_map	gid_map;
 	struct kref		kref;
 	struct user_namespace	*parent;
+	int			level;
 	kuid_t			owner;
 	kgid_t			group;
 };
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index a74dc5b..5242e83 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -39,6 +39,9 @@ int create_user_ns(struct cred *new)
 	kuid_t owner = new->euid;
 	kgid_t group = new->egid;
 
+	if (parent_ns->level > 32)
+		return -EUSERS;
+
 	/*
 	 * Verify that we can not violate the policy of which files
 	 * may be accessed that is specified by the root directory,
@@ -62,6 +65,7 @@ int create_user_ns(struct cred *new)
 
 	kref_init(&ns->kref);
 	ns->parent = parent_ns;
+	ns->level = parent_ns->level + 1;
 	ns->owner = owner;
 	ns->group = group;
 
-- 
1.7.10.4


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