cgroup: fix psi monitor for root cgroup

BugLink: https://bugs.launchpad.net/bugs/1918167
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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 35faf082a709c0dcb40462a8ab43d685b92d5af5..37db8eba149acd52ce08912539705c4a9ead56ea 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3627,6 +3627,7 @@ 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)
@@ -3635,7 +3636,8 @@ 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);