Re: [PATCH 1/1] psi: remove 500ms min window size limitation for triggers

[Kalesh Singh <kaleshsingh@google.com>]

On Wed, Mar 1, 2023 at 12:48 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Wed, Mar 1, 2023 at 12:07 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Wed, Mar 01, 2023 at 11:34:03AM -0800, Suren Baghdasaryan wrote:
> > > Current 500ms min window size for psi triggers limits polling interval
> > > to 50ms to prevent polling threads from using too much cpu bandwidth by
> > > polling too frequently. However the number of cgroups with triggers is
> > > unlimited, so this protection can be defeated by creating multiple
> > > cgroups with psi triggers (triggers in each cgroup are served by a single
> > > "psimon" kernel thread).
> > > Instead of limiting min polling period, which also limits the latency of
> > > psi events, it's better to limit psi trigger creation to authorized users
> > > only, like we do for system-wide psi triggers (/proc/pressure/* files can
> > > be written only by processes with CAP_SYS_RESOURCE capability). This also
> > > makes access rules for cgroup psi files consistent with system-wide ones.
> > > Add a CAP_SYS_RESOURCE capability check for cgroup psi file writers and
> > > remove the psi window min size limitation.
> > >
> > > Suggested-by: Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
> > > Link: https://lore.kernel.org/all/cover.1676067791.git.quic_sudaraja@quicinc.com/
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > >  kernel/cgroup/cgroup.c | 10 ++++++++++
> > >  kernel/sched/psi.c     |  4 +---
> > >  2 files changed, 11 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > > index 935e8121b21e..b600a6baaeca 100644
> > > --- a/kernel/cgroup/cgroup.c
> > > +++ b/kernel/cgroup/cgroup.c
> > > @@ -3867,6 +3867,12 @@ static __poll_t cgroup_pressure_poll(struct kernfs_open_file *of,
> > >       return psi_trigger_poll(&ctx->psi.trigger, of->file, pt);
> > >  }
> > >
> > > +static int cgroup_pressure_open(struct kernfs_open_file *of)
> > > +{
> > > +     return (of->file->f_mode & FMODE_WRITE && !capable(CAP_SYS_RESOURCE)) ?
> > > +             -EPERM : 0;
> > > +}
> >
> > I agree with the change, but it's a bit unfortunate that this check is
> > duplicated between system and cgroup.
> >
> > What do you think about psi_trigger_create() taking the file and
> > checking FMODE_WRITE and CAP_SYS_RESOURCE against file->f_cred?
>
> That's definitely doable and we don't even need to pass file to
> psi_trigger_create() since it's called only when we write to the file.
> However by moving the capability check into psi_trigger_create() we
> also postpone the check until write() instead of failing early in
> open(). I always assumed failing early is preferable but if
> consolidating the code here makes more sense then I can make the
> switch. Please let me know if you still prefer to move the check.
>

I always assumed the convention is to check the credentials on open()
per Linus comment here:

https://lore.kernel.org/r/CAHk-=whDkekE8n2LdPiKHeTdRnV--ys0V0nPZ76oPaE0fn-d+g@mail.gmail.com/#t

--Kalesh