[PATCH] user namespace: fix copy_user_ns return value

["Serge E. Hallyn" <serue@us.ibm.com>]

As found by sukadev@us.ibm.com, current -git, when compiled with
CONFIG_USER_NS=n, can be crashed by doing

#include <sched.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
	unshare(CLONE_NEWNS);
	setuid(1000);
}

as root.

The dummy version of copy_user_ns was returning NULL instead of
a valid namespace.  The following patch fixes the problem.
Please apply.

thanks,
-serge

>From 32f86740d27ef77160e438cd7dc4fcd5df159dd0 Mon Sep 17 00:00:00 2001
From: Serge E. Hallyn <serue@us.ibm.com>
Date: Tue, 17 Jul 2007 15:28:17 -0400
Subject: [PATCH] user namespace: fix copy_user_ns return value

When a CONFIG_USER_NS=n and a user tries to unshare some
namespace other than the user namespace, the dummy
copy_user_ns returns NULL rather than the old_ns.  This
value then gets assigned to task->nsproxy->user_ns, so
that a subsequent setuid, which uses task->nsproxy->user_ns,
causes a NULL pointer deref.

Fix this by returning old_ns.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
---
 include/linux/user_namespace.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index bb32057..1101b0c 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -49,7 +49,7 @@ static inline struct user_namespace *copy_user_ns(int flags,
 	if (flags & CLONE_NEWUSER)
 		return ERR_PTR(-EINVAL);
 
-	return NULL;
+	return old_ns;
 }
 
 static inline void put_user_ns(struct user_namespace *ns)
-- 
1.5.1.1.GIT