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From:   Saravana Kannan <saravanak@google.com>
Date:   Tue, 28 Feb 2023 13:57:25 -0800
Message-ID: <CAGETcx9DGK8Ass-f2wo=efHaGcQ8T2xzOqOP5TTrkM+537YPjA@mail.gmail.com>
Subject: Re: [PATCH] of: property: Add missing of_node_get() in parse_interrupt()
To:     Rob Herring <robh+dt@kernel.org>
Cc:     jjhiblot@traphandler.com, frowand.list@gmail.com,
        gregkh@linuxfoundation.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, Marc Zyngier <maz@kernel.org>
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On Tue, Feb 28, 2023 at 1:01=E2=80=AFPM Rob Herring <robh+dt@kernel.org> wr=
ote:
>
> On Tue, Feb 28, 2023 at 1:07=E2=80=AFPM Saravana Kannan <saravanak@google=
.com> wrote:
> >
> > On Tue, Feb 28, 2023 at 9:40=E2=80=AFAM Jean-Jacques Hiblot
> > <jjhiblot@traphandler.com> wrote:
> > >
> > > From: Jean Jacques Hiblot <jjhiblot@traphandler.com>
> > >
> > > As all the other parsers do, parse_interrupt() must increase the refc=
ount
> > > of the device_node. Otherwise the refcount is decremented every time
> > > parse_interrupt() is called on this node, leading to a potential
> > > use-after-free.
> > >
> > > This is a regression introduced by commit f265f06af194 ("of: property=
:
> > > Fix fw_devlink handling of interrupts/interrupts-extended"). The reas=
on is
> > > that of_irq_parse_one() does not increase the refcount while the prev=
iously
> > > used of_irq_find_parent() does.
> >
> > Thanks for catching the issue Jean!
> >
> > This feels like a bug in of_irq_parse_one() to me. It's returning a
> > reference to a node without doing a of_node_get() on it.
> >
> > Rob, Marc, Do you agree?
>
> I think you are right. If we look at the 'interrupts-extended' path,
> it just calls of_parse_phandle_with_args() which does a get.
>
> > Jean,
> >
> > If they agree, can you please fix of_irq_parse_one() and add a
> > of_node_put() to existing callers (if they aren't already doing a
> > put()).
>
> I think it is not that simple. The correct thing for callers may also
> be to hold the ref. We wouldn't want to just blindly do a put that is
> clearly wrong just to keep current behavior.

Right, I was just giving the approximate idea. If the caller keeps
using the node pointer, they shouldn't do a put().

> But not having the put
> means we're leaking refcounts as calling the APIs originally had no
> side effect. For example, IIRC, of_irq_get() is called again on each
> deferred probe. There is no of_irq_put() because Linux IRQ numbers
> aren't (or weren't?) refcounted.

Hopefully fw_devlink will avoid a lot of these deferred probes. But if
it comes to wasting memory (leaking) vs use after free, we should for
the short term switch to leaking.

IRQ themselves can't be freed once they are registered with the IRQ
framework, but I'd think the consumers can still do a get/put on an
IRQ. So, at the least, we should be able to do some put() from the
consumer context.

> Really, I'd like to get rid of exposing of_irq_parse_one() in the first p=
lace.

I don't have enough context to comment here.

-Saravana