[PATCH v2 1/2] cgroup: fix psi monitor for root cgroup

From: Odin Ugedal <odin@uged.al>
To: unlisted-recipients:; (no To-header on input)
Cc: tj@kernel.org, lizefan@huawei.com, hannes@cmpxchg.org,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
	dschatzberg@fb.com, surenb@google.com, Odin Ugedal <odin@uged.al>
Subject: [PATCH v2 1/2] cgroup: fix psi monitor for root cgroup
Date: Sat, 16 Jan 2021 18:36:33 +0100	[thread overview]
Message-ID: <20210116173634.1615875-2-odin@uged.al> (raw)
In-Reply-To: <20210116173634.1615875-1-odin@uged.al>

Fix NULL pointer dereference when adding new psi monitor to the root
cgroup. PSI files for root cgroup was introduced in df5ba5be742 by using
system wide psi struct when reading, but file write/monitor was not
properly fixed. Since the PSI config for the root cgroup isn't
initialized, the current implementation tries to lock a NULL ptr,
resulting in a crash.

Can be triggered by running this as root:
$ tee /sys/fs/cgroup/cpu.pressure <<< "some 10000 1000000"

Signed-off-by: Odin Ugedal <odin@uged.al>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
---
 kernel/cgroup/cgroup.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 613845769103..1ea995f801ec 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3564,6 +3564,7 @@ static ssize_t cgroup_pressure_write(struct kernfs_open_file *of, char *buf,
 {
 	struct psi_trigger *new;
 	struct cgroup *cgrp;
+	struct psi_group *psi;
 
 	cgrp = cgroup_kn_lock_live(of->kn, false);
 	if (!cgrp)
@@ -3572,7 +3573,8 @@ static ssize_t cgroup_pressure_write(struct kernfs_open_file *of, char *buf,
 	cgroup_get(cgrp);
 	cgroup_kn_unlock(of->kn);
 
-	new = psi_trigger_create(&cgrp->psi, buf, nbytes, res);
+	psi = cgroup_ino(cgrp) == 1 ? &psi_system : &cgrp->psi;
+	new = psi_trigger_create(psi, buf, nbytes, res);
 	if (IS_ERR(new)) {
 		cgroup_put(cgrp);
 		return PTR_ERR(new);
-- 
2.30.0

