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    Subject[PATCH 4.4 53/97] ALSA: pcm: Fix UAF at PCM release via PCM timer access
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

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

    From: Takashi Iwai <tiwai@suse.de>

    commit a820ccbe21e8ce8e86c39cd1d3bc8c7d1cbb949b upstream.

    The PCM runtime object is created and freed dynamically at PCM stream
    open / close time. This is tracked via substream->runtime, and it's
    cleared at snd_pcm_detach_substream().

    The runtime object assignment is protected by PCM open_mutex, so for
    all PCM operations, it's safely handled. However, each PCM substream
    provides also an ALSA timer interface, and user-space can access to
    this while closing a PCM substream. This may eventually lead to a
    UAF, as snd_pcm_timer_resolution() tries to access the runtime while
    clearing it in other side.

    Fortunately, it's the only concurrent access from the PCM timer, and
    it merely reads runtime->timer_resolution field. So, we can avoid the
    race by reordering kfree() and wrapping the substream->runtime
    clearance with the corresponding timer lock.

    Reported-by: syzbot+8e62ff4e07aa2ce87826@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/pcm.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    --- a/sound/core/pcm.c
    +++ b/sound/core/pcm.c
    @@ -28,6 +28,7 @@
    #include <sound/core.h>
    #include <sound/minors.h>
    #include <sound/pcm.h>
    +#include <sound/timer.h>
    #include <sound/control.h>
    #include <sound/info.h>

    @@ -1025,8 +1026,13 @@ void snd_pcm_detach_substream(struct snd
    snd_free_pages((void*)runtime->control,
    PAGE_ALIGN(sizeof(struct snd_pcm_mmap_control)));
    kfree(runtime->hw_constraints.rules);
    - kfree(runtime);
    + /* Avoid concurrent access to runtime via PCM timer interface */
    + if (substream->timer)
    + spin_lock_irq(&substream->timer->lock);
    substream->runtime = NULL;
    + if (substream->timer)
    + spin_unlock_irq(&substream->timer->lock);
    + kfree(runtime);
    put_pid(substream->pid);
    substream->pid = NULL;
    substream->pstr->substream_opened--;

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