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 11:35:16 +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>,
        Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH 4.19 062/125] ALSA: usb-audio: Fix possible NULL dereference at create_yamaha_midi_quirk()

On Wed, 13 Nov 2019 11:19:54 +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)))


thanks,

Takashi