linux-kernel - Re: [PATCH 4.4 23/97] ubi: fastmap: Dont flush fastmap work on detach

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Message-ID: <2235171.AyZ2c50PG7@blindfold>
Date:   Wed, 16 May 2018 19:37:01 +0200
From:   Richard Weinberger <richard@....at>
To:     Ben Hutchings <ben.hutchings@...ethink.co.uk>
Cc:     Martin Townsend <mtownsend1973@...il.com>, stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4.4 23/97] ubi: fastmap: Dont flush fastmap work on detach

Ben,

Am Mittwoch, 16. Mai 2018, 18:53:49 CEST schrieb Ben Hutchings:
> I don't see how this change can fix a use-after-free.  If this function
> can be called with *ubi already freed, then the rest of the function
> body is also not safe to run.  But I don't think that is the case.

thanks a lot for digging into this!
It is not about ubi (struct ubi_device) being free()'d, it is about
ubi->volumes[].

> ubi->fm_work doesn't depend on any other structure (except a global
> workqueue, which won't go away).
> 
> It seems to me that the bug is really a race condition, and removing
> the flush_work() makes it harder to hit that condition.  The proper fix
> would be to call flush_work() (or cancel_work_sync()) before the UBI
> volumes are freed.

That's a very valid point.
I think cancel_work_sync() is the right thing to do.

Thanks,
//richard