[PATCH v3 2/2] cgroup: Limit frequency of reading cgroup.stat for unprivileged users

Waiman Long
2024-07-10T18:24:26Z

Since cgroup_mutex is being acquired when reading from a world readable
cgroup.stat control file, it is possible that repeated reading of
cgroup.stat may be used as a denial of service (DoS) attack vector
by unprivileged users to greatly hinder cgroup related operations in
a system.

To prevent this, we are limiting the reading of cgroup.stat file from
unprivileged users to at most 8 times per second.

Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/cgroup/cgroup.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 18c982a06446..56ac9f14d100 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3668,12 +3668,22 @@ static int cgroup_events_show(struct seq_file *seq, void *v)

static int cgroup_stat_show(struct seq_file *seq, void *v)
{
+ static unsigned long unpriv_timestamp;
struct cgroup *cgroup = seq_css(seq)->cgroup;
struct cgroup_subsys_state *css;
int ssid;

+ /*
+ * Unprivileged users can only read cgroup.stat up to 8 times
+ * per second to avoid potential DoS attack.
+ */
+ if (!capable(CAP_SYS_ADMIN) &&
+ time_before(jiffies, unpriv_timestamp + (HZ >> 3)))
+ msleep(HZ >> 3);
+
/* cgroup_mutex required for for_each_css() */
cgroup_lock();
+ unpriv_timestamp = jiffies;
seq_printf(seq, "nr_descendants %d\n",
cgroup->nr_descendants);
seq_printf(seq, "nr_dying_descendants %d\n",
--
2.39.3