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

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On 02/11/06, Giuliano Pochini <[email protected]> wrote:
On Tue, 31 Oct 2006 23:26:31 +0100
Jesper Juhl <[email protected]> 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 <[email protected]>

--- 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 <[email protected]>
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