On Sat, Jan 16, 2021 at 06:36:33PM +0100, Odin Ugedal wrote:

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 <redacted>
Reviewed-by: Suren Baghdasaryan <redacted>

Fixes: df5ba5be7425 ("kernel/sched/psi.c: expose pressure metrics on root cgroup")
Acked-by: Johannes Weiner <redacted>

Since this is a userspace-triggerable NULL ptr crash, we should
probably also

Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org # 5.2+