linux-kernel - Re: [PATCH 4.19 062/125] ALSA: usb-audio: Fix possible NULL dereference at create_yamaha_midi

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Date:   Wed, 13 Nov 2019 11:36:45 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH 4.19 062/125] ALSA: usb-audio: Fix possible NULL dereference at create_yamaha_midi_quirk()

On Wed, 13 Nov 2019 11:35:16 +0100,
Takashi Iwai wrote:
> 
> On Wed, 13 Nov 2019 11:19:54 +0100,
> Pavel Machek wrote:
> > 
> > Hi!
> > 
> > > From: Takashi Iwai <tiwai@...e.de>
> > > 
> > > commit 60849562a5db4a1eee2160167e4dce4590d3eafe upstream.
> > > 
> > > The previous addition of descriptor validation may lead to a NULL
> > > dereference at create_yamaha_midi_quirk() when either injd or outjd is
> > > NULL.  Add proper non-NULL checks.
> > 
> > This is wrong.
> > 
> > > @@ -259,8 +259,8 @@ static int create_yamaha_midi_quirk(stru
> > >  					NULL, USB_MS_MIDI_OUT_JACK);
> > >  	if (!injd && !outjd)
> > >  		return -ENODEV;
> > 
> > Clearly code wants to allow at most one of them to be NULL.
> > 
> > > -	if (!snd_usb_validate_midi_desc(injd) ||
> > > -	    !snd_usb_validate_midi_desc(outjd))
> > > +	if (!(injd && snd_usb_validate_midi_desc(injd)) ||
> > > +	    !(outjd && snd_usb_validate_midi_desc(outjd)))
> > >  		return -ENODEV;
> > 
> > Yet it will return here if it is. Correct check would be
> > 
> > +     if (!(!injd || snd_usb_validate_midi_desc(injd)) ||
> > +         !(!outjd || snd_usb_validate_midi_desc(outjd)))
> > 
> > AFAICT.
> 
> Erm, right, but a better representation is:
> 
> 	if ((injd && !snd_usb_validate_midi_desc(injd)) ||
> 	    (outjd && !snd_usb_validate_midi_desc(injd)))

Of course, another typo:

 	if ((injd && !snd_usb_validate_midi_desc(injd)) ||
 	    (outjd && !snd_usb_validate_midi_desc(outjd)))


Takashi