On Thu, May 17, 2018 at 12:09:55AM +0200, Takashi Iwai wrote: > On Wed, 16 May 2018 22:51:47 +0200, > Ben Hutchings wrote: > > > > On Sun, 2018-04-22 at 15:53 +0200, Greg Kroah-Hartman wrote: > > > 4.4-stable review patch. If anyone has any objections, please let me know. > > > > > > ------------------ > > > > > > From: Takashi Iwai <tiwai@xxxxxxx> > > > > > > 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. > > [...] > > > > This seems to depend on: > > > > commit f65e0d299807d8a11812845c972493c3f9a18e10 > > Author: Takashi Iwai <tiwai@xxxxxxx> > > Date: Wed Feb 10 12:47:03 2016 +0100 > > > > ALSA: timer: Call notifier in the same spinlock > > > > (But I'm not totally convinced that snd_pcm_timer_resolution() is > > always called with the timer lock held, even after that.) Good catch, I'll queue this up now, thanks. greg k-h
