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    Subject[PATCH 4.16 112/196] ALSA: pcm: Fix endless loop for XRUN recovery in OSS emulation
    Date
    4.16-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Takashi Iwai <tiwai@suse.de>

    commit e15dc99dbb9cf99f6432e8e3c0b3a8f7a3403a86 upstream.

    The commit 02a5d6925cd3 ("ALSA: pcm: Avoid potential races between OSS
    ioctls and read/write") split the PCM preparation code to a locked
    version, and it added a sanity check of runtime->oss.prepare flag
    along with the change. This leaded to an endless loop when the stream
    gets XRUN: namely, snd_pcm_oss_write3() and co call
    snd_pcm_oss_prepare() without setting runtime->oss.prepare flag and
    the loop continues until the PCM state reaches to another one.

    As the function is supposed to execute the preparation
    unconditionally, drop the invalid state check there.

    The bug was triggered by syzkaller.

    Fixes: 02a5d6925cd3 ("ALSA: pcm: Avoid potential races between OSS ioctls and read/write")
    Reported-by: syzbot+150189c103427d31a053@syzkaller.appspotmail.com
    Reported-by: syzbot+7e3f31a52646f939c052@syzkaller.appspotmail.com
    Reported-by: syzbot+4f2016cf5185da7759dc@syzkaller.appspotmail.com
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Takashi Iwai <tiwai@suse.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    sound/core/oss/pcm_oss.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    --- a/sound/core/oss/pcm_oss.c
    +++ b/sound/core/oss/pcm_oss.c
    @@ -1128,13 +1128,14 @@ static int snd_pcm_oss_get_active_substr
    }

    /* call with params_lock held */
    +/* NOTE: this always call PREPARE unconditionally no matter whether
    + * runtime->oss.prepare is set or not
    + */
    static int snd_pcm_oss_prepare(struct snd_pcm_substream *substream)
    {
    int err;
    struct snd_pcm_runtime *runtime = substream->runtime;

    - if (!runtime->oss.prepare)
    - return 0;
    err = snd_pcm_kernel_ioctl(substream, SNDRV_PCM_IOCTL_PREPARE, NULL);
    if (err < 0) {
    pcm_dbg(substream->pcm,

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