linux-kernel - Re: [PATCH 6.12.y 6.6.y 6.1.y 5.15.y 5.10.y 5.4.y] ALSA: usb-audio: Kill timer properly at removal

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Message-ID: <CAO9qdTEo46hCZ0UXKjjBQ4W2sLT2+5zw9DugQF98sqpHLyNzPg@mail.gmail.com>
Date: Thu, 9 Oct 2025 20:23:42 +0900
From: Jeongjun Park <aha310510@...il.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: stable@...r.kernel.org, Takashi Iwai <tiwai@...e.de>, 
	syzbot+d8f72178ab6783a7daea@...kaller.appspotmail.com, 
	Clemens Ladisch <clemens@...isch.de>, Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org, 
	linux-sound@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6.12.y 6.6.y 6.1.y 5.15.y 5.10.y 5.4.y] ALSA: usb-audio:
 Kill timer properly at removal

Hi Greg,

Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Wed, Oct 08, 2025 at 12:58:08AM +0900, Jeongjun Park wrote:
> > From: Takashi Iwai <tiwai@...e.de>
> >
> > [ Upstream commit 0718a78f6a9f04b88d0dc9616cc216b31c5f3cf1 ]
> >
> > The USB-audio MIDI code initializes the timer, but in a rare case, the
> > driver might be freed without the disconnect call.  This leaves the
> > timer in an active state while the assigned object is released via
> > snd_usbmidi_free(), which ends up with a kernel warning when the debug
> > configuration is enabled, as spotted by fuzzer.
> >
> > For avoiding the problem, put timer_shutdown_sync() at
> > snd_usbmidi_free(), so that the timer can be killed properly.
> > While we're at it, replace the existing timer_delete_sync() at the
> > disconnect callback with timer_shutdown_sync(), too.
> >
> > Reported-by: syzbot+d8f72178ab6783a7daea@...kaller.appspotmail.com
> > Closes: https://lore.kernel.org/681c70d7.050a0220.a19a9.00c6.GAE@google.com
> > Cc: <stable@...r.kernel.org>
> > Link: https://patch.msgid.link/20250519212031.14436-1-tiwai@suse.de
> > Signed-off-by: Takashi Iwai <tiwai@...e.de>
> > [ del_timer vs timer_delete differences ]
> > Signed-off-by: Jeongjun Park <aha310510@...il.com>
> > ---
> >  sound/usb/midi.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/sound/usb/midi.c b/sound/usb/midi.c
> > index a792ada18863..c3de2b137435 100644
> > --- a/sound/usb/midi.c
> > +++ b/sound/usb/midi.c
> > @@ -1530,6 +1530,7 @@ static void snd_usbmidi_free(struct snd_usb_midi *umidi)
> >                       snd_usbmidi_in_endpoint_delete(ep->in);
> >       }
> >       mutex_destroy(&umidi->mutex);
> > +     timer_shutdown_sync(&umidi->error_timer);
>
> This function is not in older kernel versions, you did not test this
> build :(
>
> I've applied this to 6.6.y and newer, but for 6.1.y and older, please
> use the proper function.

Sorry, I didn't realize that the timer_shutdown_sync() implementation
commit wasn't backported to versions prior to 6.2.

But why wasn't this feature backported to previous versions? I understand
that most drivers write separate patches for pre-6.2 versions that don't
implement timer_shutdown_sync() to address this type of bug.

Is there some other, unavoidable reason I'm unaware of?

>
> thanks,
>
> greg k-h

Regards,
Jeongjun Park