[PATCH review 3/6] userns: Recommend use of memory control groups.

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

In the help text describing user namespaces recommend use of memory
control groups.  In many cases memory control groups are the only
mechanism there is to limit how much memory a user who can create
user namespaces can use.

Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
---
 Documentation/namespaces/resource-control.txt |   10 ++++++++++
 init/Kconfig                                  |    7 +++++++
 2 files changed, 17 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/namespaces/resource-control.txt

diff --git a/Documentation/namespaces/resource-control.txt b/Documentation/namespaces/resource-control.txt
new file mode 100644
index 0000000..3d8178a
--- /dev/null
+++ b/Documentation/namespaces/resource-control.txt
@@ -0,0 +1,10 @@
+There are a lot of kinds of objects in the kernel that don't have
+individual limits or that have limits that are ineffective when a set
+of processes is allowed to switch user ids.  With user namespaces
+enabled in a kernel for people who don't trust their users or their
+users programs to play nice this problems becomes more acute.
+
+Therefore it is recommended that memory control groups be enabled in
+kernels that enable user namespaces, and it is further recommended
+that userspace configure memory control groups to limit how much
+memory users they don't trust to play nice can use.
diff --git a/init/Kconfig b/init/Kconfig
index 7d30240..c8c58bd 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1035,6 +1035,13 @@ config USER_NS
 	help
 	  This allows containers, i.e. vservers, to use user namespaces
 	  to provide different user info for different servers.
+
+	  When user namespaces are enabled in the kernel it is
+	  recommended that the MEMCG and MEMCG_KMEM options also be
+	  enabled and that user-space use the memory control groups to
+	  limit the amount of memory a memory unprivileged users can
+	  use.
+
 	  If unsure, say N.
 
 config PID_NS
-- 
1.7.5.4