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    Subject[PATCH 4.9 06/36] ALSA: line6: Fix racy initialization of LINE6 MIDI
    Date
    From: Takashi Iwai <tiwai@suse.de>

    commit 05ca447630334c323c9e2b788b61133ab75d60d3 upstream.

    The initialization of MIDI devices that are found on some LINE6
    drivers are currently done in a racy way; namely, the MIDI buffer
    instance is allocated and initialized in each private_init callback
    while the communication with the interface is already started via
    line6_init_cap_control() call before that point. This may lead to
    Oops in line6_data_received() when a spurious event is received, as
    reported by syzkaller.

    This patch moves the MIDI initialization to line6_init_cap_control()
    as well instead of the too-lately-called private_init for avoiding the
    race. Also this reduces slightly more lines, so it's a win-win
    change.

    Reported-by: syzbot+0d2b3feb0a2887862e06@syzkallerlkml..appspotmail.com
    Link: https://lore.kernel.org/r/000000000000a4be9405c28520de@google.com
    Link: https://lore.kernel.org/r/20210517132725.GA50495@hyeyoo
    Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
    Cc: <stable@vger.kernel.org>
    Link: https://lore.kernel.org/r/20210518083939.1927-1-tiwai@suse.de
    Signed-off-by: Takashi Iwai <tiwai@suse.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    sound/usb/line6/driver.c | 4 ++++
    sound/usb/line6/pod.c | 5 -----
    sound/usb/line6/variax.c | 6 ------
    3 files changed, 4 insertions(+), 11 deletions(-)

    --- a/sound/usb/line6/driver.c
    +++ b/sound/usb/line6/driver.c
    @@ -687,6 +687,10 @@ static int line6_init_cap_control(struct
    line6->buffer_message = kmalloc(LINE6_MIDI_MESSAGE_MAXLEN, GFP_KERNEL);
    if (!line6->buffer_message)
    return -ENOMEM;
    +
    + ret = line6_init_midi(line6);
    + if (ret < 0)
    + return ret;
    } else {
    ret = line6_hwdep_init(line6);
    if (ret < 0)
    --- a/sound/usb/line6/pod.c
    +++ b/sound/usb/line6/pod.c
    @@ -421,11 +421,6 @@ static int pod_init(struct usb_line6 *li
    if (err < 0)
    return err;

    - /* initialize MIDI subsystem: */
    - err = line6_init_midi(line6);
    - if (err < 0)
    - return err;
    -
    /* initialize PCM subsystem: */
    err = line6_init_pcm(line6, &pod_pcm_properties);
    if (err < 0)
    --- a/sound/usb/line6/variax.c
    +++ b/sound/usb/line6/variax.c
    @@ -217,7 +217,6 @@ static int variax_init(struct usb_line6
    const struct usb_device_id *id)
    {
    struct usb_line6_variax *variax = (struct usb_line6_variax *) line6;
    - int err;

    line6->process_message = line6_variax_process_message;
    line6->disconnect = line6_variax_disconnect;
    @@ -233,11 +232,6 @@ static int variax_init(struct usb_line6
    if (variax->buffer_activate == NULL)
    return -ENOMEM;

    - /* initialize MIDI subsystem: */
    - err = line6_init_midi(&variax->line6);
    - if (err < 0)
    - return err;
    -
    /* initiate startup procedure: */
    variax_startup1(variax);
    return 0;

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