linux-kernel - Re: [PATCH v2 3/3] sched/psi: allow unprivileged polling of N*2s period

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Date:   Thu, 23 Mar 2023 11:07:25 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        brauner@...nel.org, chris@...isdown.name
Subject: Re: [PATCH v2 3/3] sched/psi: allow unprivileged polling of N*2s period

On Thu, Mar 23, 2023 at 10:45 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Thu, Mar 23, 2023 at 09:55:11AM -0700, Suren Baghdasaryan wrote:
> > On Thu, Mar 23, 2023 at 3:34 AM Domenico Cerasuolo
> > > @@ -1254,16 +1262,19 @@ int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
> > >  }
> > >
> > >  struct psi_trigger *psi_trigger_create(struct psi_group *group,
> > > -                       char *buf, enum psi_res res)
> > > +                       char *buf, enum psi_res res, struct file *file)
> > >  {
> > >         struct psi_trigger *t;
> > >         enum psi_states state;
> > >         u32 threshold_us;
> > > +       bool privileged;
> > >         u32 window_us;
> > >
> > >         if (static_branch_likely(&psi_disabled))
> > >                 return ERR_PTR(-EOPNOTSUPP);
> > >
> > > +       privileged = cap_raised(file->f_cred->cap_effective, CAP_SYS_RESOURCE);
> >
> > I missed one detail here. We are moving the cap check from open() to
> > write(). That might break potential users which open the file from a
> > process with that cap and then pass that FD to an unprivileged process
> > to create the trigger by writing to that file. I'm not aware of any
> > use of such a pattern but it is possible there are such users.
> > With this change such users would have to delegate trigger creation to
> > the privileged process too and the received FD would be used only for
> > polling. IMHO that's a safer pattern because triggers are created by
> > the privileged process.
>
> Oh, it's checking file->f_cred, which is set up at open(). So if the
> opener is privileged, the write can be delegated to an unprivileged
> process.
>
> But I agree that this is subtle and could use a comment.
>
> This was a usecase specifically requested by Christian, actually.

Ah, great!