linux-kernel - Re: [PATCH] genirq: fix reference leaks on irq affinity notifiers

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Date:   Tue, 17 Mar 2020 14:56:46 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Edward Cree <ecree@...arflare.com>, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, psodagud@...eaurora.org
Subject: Re: [PATCH] genirq: fix reference leaks on irq affinity notifiers

On Tue, 2020-03-17 at 10:58 +0000, Edward Cree wrote:
> On 15/03/2020 20:29, Ben Hutchings wrote:
> > ...since the pending work item holds a reference to the notification
> > state, it's still not clear to me why or whether "genirq: Prevent use-
> > after-free and work list corruption" was needed.
> Yeah, I think that commit was bogus.  The email thread[1] doesn't
>  exactly inspire confidence either.  I think the submitter just didn't
>  realise that there was a ref corresponding to the work; AFAICT there's
>  no way the alleged "work list corruption" could happen.
> 
> > If it's reasonable to cancel_work_sync() when removing a notifier, I
> > think we can remove the kref and call the release function directly.
> I'd prefer to stick to the smaller fix for -rc and stable.  But if you
>  want to remove the kref for -next, I'd be happy to Ack that patch.

OK, then you can add:

Acked-by: Ben Hutchings <ben@...adent.org.uk>

to this one.

> Btw, we (sfc linux team) think there's still a use-after-free issue in
>  the cpu_rmap lib, as follows:
> 1) irq_cpu_rmap_add creates a glue and notifier, adds glue to rmap->obj
> 2) someone else does irq_set_affinity_notifier.
>    This causes cpu_rmap's notifier (old_notify) to get released, and so
>    irq_cpu_rmap_release kfrees glue.  But it's still in rmap->obj
> 3) free_irq_cpu_rmap loops over obj, finds the glue, tries to clear its
>    notifier.
> Now one could say that this UAF is academic, since having two bits of
>  code trying to register notifiers for the same IRQ is broken anyway
>  (in this case, the rmap would stop getting updated, because the
>  "someone else" stole the notifier).

So far as I can remember, my thinking was that only non-shared IRQs
will have notifiers and only the current user of the IRQ will set the
notifier.  The doc comment for irq_set_affinity_notifier() implies the
latter restriction, but it might be worth spelling this out explicitly.

Ben.

> But I thought I'd bring it up in case it's halfway relevant.
> 
> -ed
> 
> [1] https://lore.kernel.org/lkml/1553119211-29761-1-git-send-email-psodagud@codeaurora.org/T/#u
-- 
Ben Hutchings
For every complex problem
there is a solution that is simple, neat, and wrong.


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