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: <CAO9qdTGktp71We9BAhsd3bewb9yKWyOjbnq2TQh0CaV7CqD6UA@mail.gmail.com>
Date: Thu, 9 Oct 2025 21:42:04 +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 Thu, Oct 09, 2025 at 08:23:42PM +0900, Jeongjun Park wrote:
> > 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.
>
> Features are not backported to older kernels.  If you want this fix in
> an older kernel, then you need to either backport that feature, or fix
> up the driver to use whatever was used before that function existed.

I could write a separate patch to fix this bug, but I'm not happy that
great features like timer_shutdown_sync() haven't been backported.

So, I'll try to backport the "timers: Provide timer_shutdown[_sync]()"
patch series sometime soon.

>
> thanks,
>
> greg k-h

Regards,
Jeongjun Park