commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
author Eric W. Biederman <ebiederm@xmission.com> Wed Mar 13 11:51:49 2013 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> Wed Mar 13 15:00:20 2013 -0700
tree 768e1d799f6d6a4a7f85f48d9e9fc431dc8d017c
parent 6c23cbbd5056b155401b0a2b5567d530e6c750c4

userns: Don't allow CLONE_NEWUSER | CLONE_FS

Don't allowing sharing the root directory with processes in a
different user namespace.  There doesn't seem to be any point, and to
allow it would require the overhead of putting a user namespace
reference in fs_struct (for permission checks) and incrementing that
reference count on practically every call to fork.

So just perform the inexpensive test of forbidding sharing fs_struct
acrosss processes in different user namespaces.  We already disallow
other forms of threading when unsharing a user namespace so this
should be no real burden in practice.

This updates setns, clone, and unshare to disallow multiple user
namespaces sharing an fs_struct.

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/user_namespace.c

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 8b65083..b14f4d3 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -21,6 +21,7 @@
 #include <linux/uaccess.h>
 #include <linux/ctype.h>
 #include <linux/projid.h>
+#include <linux/fs_struct.h>
 
 static struct kmem_cache *user_ns_cachep __read_mostly;
 
@@ -837,6 +838,9 @@
 	if (atomic_read(&current->mm->mm_users) > 1)
 		return -EINVAL;
 
+	if (current->fs->users != 1)
+		return -EINVAL;
+
 	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
 		return -EPERM;