cgroup: fix psi monitor for root cgroup

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>
Acked-by: Dan Schatzberg <dschatzberg@fb.com>
Fixes: df5ba5be7425 ("kernel/sched/psi.c: expose pressure metrics on root cgroup")
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: stable@vger.kernel.org # 5.2+
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 613845769103cd36ab9ef9340b9dd90f5c20f5f9..1ea995f801ecfe36098a6e3f08c2dd3e9c2394c2 100644 (file)
--- 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);