cgroup: pids: fix invalid get/put usage

Fix incorrect usage of css_get and css_put to put a different css in
pids_{cancel_,}attach() than the one grabbed in pids_can_attach(). This
could lead to quite serious memory leakage (and unsafe operations on the
putted css).

tj: minor comment update
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup_pids.c

@@ -173,11 +173,11 @@ static int pids_can_attach(struct cgroup_subsys_state *css,
 		struct pids_cgroup *old_pids;
 
 		/*
-		 * Grab a ref to each task's css. We don't drop the ref until
-		 * we either fail and hit ->cancel_attach() or succeed and hit
-		 * ->attach().
+		 * No need to pin @old_css between here and cancel_attach()
+		 * because cgroup core protects it from being freed before
+		 * the migration completes or fails.
 		 */
-		old_css = task_get_css(task, pids_cgrp_id);
+		old_css = task_css(task, pids_cgrp_id);
 		old_pids = css_pids(old_css);
 
 		pids_charge(pids, 1);
@@ -202,19 +202,9 @@ static void pids_cancel_attach(struct cgroup_subsys_state *css,
 
 		pids_charge(old_pids, 1);
 		pids_uncharge(pids, 1);
-		css_put(old_css);
 	}
 }
 
-static void pids_attach(struct cgroup_subsys_state *css,
-			struct cgroup_taskset *tset)
-{
-	struct task_struct *task;
-
-	cgroup_taskset_for_each(task, tset)
-		css_put(task_css(task, pids_cgrp_id));
-}
-
 static int pids_can_fork(struct task_struct *task, void **priv_p)
 {
 	struct cgroup_subsys_state *css;
@@ -354,7 +344,6 @@ static struct cftype pids_files[] = {
 struct cgroup_subsys pids_cgrp_subsys = {
 	.css_alloc	= pids_css_alloc,
 	.css_free	= pids_css_free,
-	.attach		= pids_attach,
 	.can_attach 	= pids_can_attach,
 	.cancel_attach 	= pids_cancel_attach,
 	.can_fork	= pids_can_fork,