From: Takashi Iwai Date: Mon, 2 Apr 2018 20:41:43 +0000 (+0200) Subject: ALSA: pcm: Fix UAF at PCM release via PCM timer access X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=0bb5579128e65d8c4334f90ca47cfe49e90187f7;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git ALSA: pcm: Fix UAF at PCM release via PCM timer access 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: Signed-off-by: Takashi Iwai Signed-off-by: Greg Kroah-Hartman --- diff --git a/sound/core/pcm.c b/sound/core/pcm.c index 7fea724d093a..e8dc1a5afe66 100644 --- a/sound/core/pcm.c +++ b/sound/core/pcm.c @@ -28,6 +28,7 @@ #include #include #include +#include #include #include @@ -1050,8 +1051,13 @@ void snd_pcm_detach_substream(struct snd_pcm_substream *substream) 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--;