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

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Date:   Tue, 19 Jan 2021 11:52:37 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Odin Ugedal <odin@...d.al>
Cc:     tj@...nel.org, lizefan@...wei.com, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, dschatzberg@...com, surenb@...gle.com
Subject: Re: [PATCH v2 1/2] cgroup: fix psi monitor for root cgroup

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@...d.al>
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

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

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

Cc: stable@...r.kernel.org # 5.2+