[PATCH] memcg: limit the number of thresholds per-memcg

[Michal Hocko]

There is no limit for the maximum number of threshold events registered
per memcg. It is even worse that all the events are stored in a
per-memcg table which is enlarged when a new event is registered. This
can lead to the following issue mentioned by Tejun:
"
So, let's say the file is write-accessible to !priv user which is
under reasonable resource limits.  Normally this shouldn't affect priv
system tools which are monitoring the same event as it shouldn't be
able to deplete resources as long as the resource control mechanisms
are configured and functioning properly; however, the memory usage
event puts all event listeners into a single contiguous table which a
!priv user can easily expand to a size where the table can no longer
be enlarged and if a priv system tool or another user tries to
register event afterwards, it'll fail.  IOW, it creates a shared
resource which isn't properly provisioned and can be trivially filled
up making it an easy DoS target.
"

Let's be more strict and cap the number of events that might be
registered. MAX_THRESHOLD_EVENTS value is more or less random. The
expectation is that it should be high enough to cover reasonable
usecases while not too high to allow excessive resources consumption.
1024 events consume something like 16KB which shouldn't be a big deal
and it should be good enough.

Reported-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/memcontrol.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e4330cd..8247db3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5401,6 +5401,9 @@ static void mem_cgroup_oom_notify(struct mem_cgroup *memcg)
 		mem_cgroup_oom_notify_cb(iter);
 }
 
+/* Maximum number of treshold events registered per memcg. */
+#define MAX_THRESHOLD_EVENTS	1024
+
 static int mem_cgroup_usage_register_event(struct cgroup *cgrp,
 	struct cftype *cft, struct eventfd_ctx *eventfd, const char *args)
 {
@@ -5424,6 +5427,11 @@ static int mem_cgroup_usage_register_event(struct cgroup *cgrp,
 	else
 		BUG();
 
+	if (thresholds->primary->size == MAX_THRESHOLD_EVENTS) {
+		ret = -ENOSPC;
+		goto unlock;
+	}
+
 	usage = mem_cgroup_usage(memcg, type == _MEMSWAP);
 
 	/* Check if a threshold crossed before adding a new one */
-- 
1.7.10.4

-- 
Michal Hocko
SUSE Labs
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