cgroup: always put cset in cgroup_css_set_put_fork

A successful call to cgroup_css_set_fork() will always have taken
a ref on kargs->cset (regardless of CLONE_INTO_CGROUP), so always
do a corresponding put in cgroup_css_set_put_fork().

Without this, a cset and its contained css structures will be
leaked for some fork failures.  The following script reproduces
the leak for a fork failure due to exceeding pids.max in the
pids controller.  A similar thing can happen if we jump to the
bad_fork_cancel_cgroup label in copy_process().

[ -z "$1" ] && echo "Usage $0 pids-root" && exit 1
PID_ROOT=$1
CGROUP=$PID_ROOT/foo

[ -e $CGROUP ] && rmdir -f $CGROUP
mkdir $CGROUP
echo 5 > $CGROUP/pids.max
echo $$ > $CGROUP/cgroup.procs

fork_bomb()
{
	set -e
	for i in $(seq 10); do
		/bin/sleep 3600 &
	done
}

(fork_bomb) &
wait
echo $$ > $PID_ROOT/cgroup.procs
kill $(cat $CGROUP/cgroup.procs)
rmdir $CGROUP

Fixes: ef2c41cf38a7 ("clone3: allow spawning processes into cgroups")
Cc: stable@vger.kernel.org # v5.7+
Signed-off-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 625d7483951ce94d99c478de8ea4958ca46d51be..245cf62ce85aee10bbc0e9e436c2334be9fdc088 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6486,19 +6486,18 @@ err:
 static void cgroup_css_set_put_fork(struct kernel_clone_args *kargs)
        __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
 {
+       struct cgroup *cgrp = kargs->cgrp;
+       struct css_set *cset = kargs->cset;
+
        cgroup_threadgroup_change_end(current);
 
-       if (kargs->flags & CLONE_INTO_CGROUP) {
-               struct cgroup *cgrp = kargs->cgrp;
-               struct css_set *cset = kargs->cset;
+       if (cset) {
+               put_css_set(cset);
+               kargs->cset = NULL;
+       }
 
+       if (kargs->flags & CLONE_INTO_CGROUP) {
                cgroup_unlock();
-
-               if (cset) {
-                       put_css_set(cset);
-                       kargs->cset = NULL;
-               }
-
                if (cgrp) {
                        cgroup_put(cgrp);
                        kargs->cgrp = NULL;