linux-kernel - Re: [PATCH v2] netlink: do not set cb

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Date:   Mon, 09 Oct 2017 14:31:29 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>, davem@...emloft.net,
        Netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] netlink: do not set cb_running if dump's start() errs

On Mon, 2017-10-09 at 14:14 +0200, Jason A. Donenfeld wrote:
> It turns out that multiple places can call netlink_dump(), which
> means
> it's still possible to dereference partially initialized values in
> dump() that were the result of a faulty returned start().
> 
> This fixes the issue by calling start() _before_ setting cb_running
> to
> true, so that there's no chance at all of hitting the dump() function
> through any indirect paths.
> 
> It also moves the call to start() to be when the mutex is held. This
> has
> the nice side effect of serializing invocations to start(), which is
> likely desirable anyway. It also prevents any possible other races
> that
> might come out of this logic.

I'm not necessarily sure it's _nice_, but I do think it doesn't matter,
so that's just splitting hairs. If you do have a genl family with
parallel_ops, you'd better be prepared to handle parallel things, and
then this could also be in parallel :-)

> In testing this with several different pieces of tricky code to
> trigger
> these issues, this commit fixes all avenues that I'm aware of.
> 
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>

Reviewed-by: Johannes Berg <johannes@...solutions.net>

johannes