proc: Set correct userns for new proc super created by a new pid_namespace

Fix the setting up a new proc superblock for a new pid_namespace such that
the user_ns for that proc superblock needs to be taken from the new
pid_namespace and not the active process.

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>

fs/proc/root.c

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 1d6e5bf..1419b48 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -315,6 +315,11 @@ int pid_ns_prepare_proc(struct pid_namespace *ns)
 	if (IS_ERR(fc))
 		return PTR_ERR(fc);
 
+	if (fc->user_ns != ns->user_ns) {
+		put_user_ns(fc->user_ns);
+		fc->user_ns = get_user_ns(ns->user_ns);
+	}
+
 	ctx = fc->fs_private;
 	if (ctx->pid_ns != ns) {
 		put_pid_ns(ctx->pid_ns);