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 df5ba5b 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 <[email protected]>
Reviewed-by: Suren Baghdasaryan <[email protected]>
Acked-by: Dan Schatzberg <[email protected]>
Fixes: df5ba5b ("kernel/sched/psi.c: expose pressure metrics on root cgroup")
Acked-by: Johannes Weiner <[email protected]>
Cc: [email protected] # 5.2+
Signed-off-by: Tejun Heo <[email protected]>

Author: odinuge

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);