Re: [PATCH v2 1/2] cgroup: fix psi monitor for root cgroup

[Johannes Weiner]

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 <odin@xxxxxxx>
> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

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

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

Cc: stable@xxxxxxxxxxxxxxx # 5.2+