[PATCH 5.10 010/104] cgroup: fix psi monitor for root cgroup

[Greg Kroah-Hartman]

From: Odin Ugedal <odin@xxxxxxx>

commit 385aac1519417b89cb91b77c22e4ca21db563cd0 upstream.

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@xxxxxxx>
Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Acked-by: Dan Schatzberg <dschatzberg@xxxxxx>
Fixes: df5ba5be7425 ("kernel/sched/psi.c: expose pressure metrics on root cgroup")
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 5.2+
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/cgroup/cgroup.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3567,6 +3567,7 @@ static ssize_t cgroup_pressure_write(str
 {
 	struct psi_trigger *new;
 	struct cgroup *cgrp;
+	struct psi_group *psi;
 
 	cgrp = cgroup_kn_lock_live(of->kn, false);
 	if (!cgrp)
@@ -3575,7 +3576,8 @@ static ssize_t cgroup_pressure_write(str
 	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);