linux-kernel - Re: [PATCH] sched: psi: fix unprivileged polling against cgroups

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Date:   Thu, 26 Oct 2023 13:01:50 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Luca Boccassi <bluca@...ian.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] sched: psi: fix unprivileged polling against cgroups

On Thu, Oct 26, 2023 at 09:55:23AM -0700, Suren Baghdasaryan wrote:
> On Thu, Oct 26, 2023 at 9:49 AM Luca Boccassi <bluca@...ian.org> wrote:
> >
> > On Thu, 26 Oct 2023 at 17:41, Johannes Weiner <hannes@...xchg.org> wrote:
> > >
> > > 519fabc7aaba ("psi: remove 500ms min window size limitation for
> > > triggers") breaks unprivileged psi polling on cgroups.
> > >
> > > Historically, we had a privilege check for polling in the open() of a
> > > pressure file in /proc, but were erroneously missing it for the open()
> > > of cgroup pressure files.
> > >
> > > When unprivileged polling was introduced in d82caa273565 ("sched/psi:
> > > Allow unprivileged polling of N*2s period"), it needed to filter
> > > privileges depending on the exact polling parameters, and as such
> > > moved the CAP_SYS_RESOURCE check from the proc open() callback to
> > > psi_trigger_create(). Both the proc files as well as cgroup files go
> > > through this during write(). This implicitly added the missing check
> > > for privileges required for HT polling for cgroups.
> > >
> > > When 519fabc7aaba ("psi: remove 500ms min window size limitation for
> > > triggers") followed right after to remove further restrictions on the
> > > RT polling window, it incorrectly assumed the cgroup privilege check
> > > was still missing and added it to the cgroup open(), mirroring what we
> > > used to do for proc files in the past.
> > >
> > > As a result, unprivileged poll requests that would be supported now
> > > get rejected when opening the cgroup pressure file for writing.
> 
> Ah, I see the problem. In our discussion
> https://lore.kernel.org/all/ZADj4YX4uftK%2FFrh@cmpxchg.org/ we decided
> to have the check in open() to fail early but we never considered
> unprivileged processes which only poll and never create any triggers.
> Makes sense.

Yeah, the two patches just ended up clashing. We made that open()
decision before unprivileged polling was merged, then ended up merging
it before the window patch.

Thanks!