linux-kernel - Re: [PATCH] ALSA: usb-audio: Fix memory leak in __snd_usbmidi

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Date:   Mon, 28 Oct 2019 17:38:56 +0100
From:   Clemens Ladisch <clemens@...isch.de>
To:     Navid Emamdoost <navid.emamdoost@...il.com>
Cc:     Takashi Iwai <tiwai@...e.de>, Navid Emamdoost <emamd001@....edu>,
        Kangjie Lu <kjlu@....edu>, Stephen McCamant <smccaman@....edu>,
        Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ALSA: usb-audio: Fix memory leak in __snd_usbmidi_create

Navid Emamdoost wrote:
> On Mon, Oct 28, 2019 at 1:27 AM Takashi Iwai <tiwai@...e.de> wrote:
>> On Sun, 27 Oct 2019 23:10:06 +0100,
>> Navid Emamdoost wrote:
>>> In the implementation of __snd_usbmidi_create() there is a memory leak
>>> caused by incorrect goto destination. Go to free_midi if
>>> snd_usbmidi_create_endpoints_midiman() or snd_usbmidi_create_endpoints()
>>> fail.
>>
>> No, this will lead to double-free.  After registering the rawmidi
>> interface at snd_usbmidi_create_rawmidi(), the common destructor will
>> be called via rawmidi private_free callback, and this will release the
>> all resources already.
>
> Now I can see how rawmidi private_free is set up to release the
> resources, but what concerns me is that at the moment of endpoint/port
> creation umidi is not yet added to the midi_list.
> In other words, what I see is that we still have just one local
> pointer to umidi if any of snd_usbmidi_create_endpoint* fail.

The snd_rawmidi device is automatically registered with the card, so the
line "rmidi->private_free = snd_usbmidi_rawmidi_free;" is the exact point
where the ownership of umidi changes.  midi_list does not matter.


Regards,
Clemens