linux-kernel - Re: [PATCH] Fix potential NULL pointer dereference in echoaudio midi.

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Date:	Thu, 2 Nov 2006 21:22:43 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Giuliano Pochini" <pochini@...ny.it>
Cc:	rientjes@...washington.edu, linux-kernel@...r.kernel.org,
	tiwai@...e.de
Subject: Re: [PATCH] Fix potential NULL pointer dereference in echoaudio midi.

On 02/11/06, Giuliano Pochini <pochini@...ny.it> wrote:
> On Tue, 31 Oct 2006 23:26:31 +0100
> Jesper Juhl <jesper.juhl@...il.com> wrote:
>
> > On Tuesday 31 October 2006 23:13, David Rientjes wrote:
> > > On Tue, 31 Oct 2006, Jesper Juhl wrote:
> > >
> > > > In sound/pci/echoaudio/midi.c::snd_echo_midi_output_write(), there's a risk
> > > > of dereferencing a NULL 'chip->midi_out'.
> > > > This patch contains the obvious fix as also used a bit higher up in the
> > > > same function.
> > > >
> > >
> > > How about just adding an early test:
> > >     if (!chip->midi_out)
> > >             goto out;
>
>
> The point of that check is to make sure is doesn't access chip->midi_out
> when (surprise!) it is NULL. This can only happen in the rare (possible?)
> case snd_echo_midi_output_close() is called while the timer handler is
> running. I have another proposal which IMHO is smp-safer that just moving
> the check. In that case we should also put a spinlock around the
> chip->midi_out=0 in the snd_echo_midi_output_close() callback.
>
>
> Signed-off-by: Giuliano Pochini <pochini@...ny.it>
>
> --- alsa-kernel/pci/echoaudio/midi.c__orig      2006-11-02 20:39:45.000000000 +0100
> +++ alsa-kernel/pci/echoaudio/midi.c    2006-11-02 20:44:22.000000000 +0100
> @@ -213,7 +213,7 @@ static void snd_echo_midi_output_write(u
>         sent = bytes = 0;
>         spin_lock_irqsave(&chip->lock, flags);
>         chip->midi_full = 0;
> -       if (chip->midi_out && !snd_rawmidi_transmit_empty(chip->midi_out)) {
> +       if (!snd_rawmidi_transmit_empty(chip->midi_out)) {
>                 bytes = snd_rawmidi_transmit_peek(chip->midi_out, buf,
>                                                   MIDI_OUT_BUFFER_SIZE - 1);
>                 DE_MID(("Try to send %d bytes...\n", bytes));
> @@ -264,9 +264,11 @@ static void snd_echo_midi_output_trigger
>                 }
>         } else {
>                 if (chip->tinuse) {
> -                       del_timer(&chip->timer);
>                         chip->tinuse = 0;
> +                       spin_unlock_irq(&chip->lock);
> +                       del_timer_sync(&chip->timer);
>                         DE_MID(("Timer removed\n"));
> +                       return;
>                 }
>         }
>         spin_unlock_irq(&chip->lock);
>

Fine by me.
Let's just get one of the versions pushed into -mm or mainline :)

-- 
Jesper Juhl <jesper.juhl@...il.com>
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