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: NSerge E. Hallyn <serue@us.ibm.com> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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