Re: [PATCH 1/1] ALSA: usb-audio: Fix UAF decrement if card has no live interfaces in card.c

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, May 22, 2019 at 10:26:55AM -0700, Suren Baghdasaryan wrote:
> Commit 5f8cf712582617d523120df67d392059eaf2fc4b upstream.
> 
> This is a backport to stable 3.18.y. Implementation in 3.18 differs using
> chip->probing flag instead of chip->active atomic but it still has the UAF
> issue.
> 
> If a USB sound card reports 0 interfaces, an error condition is triggered
> and the function usb_audio_probe errors out. In the error path, there was a
> use-after-free vulnerability where the memory object of the card was first
> freed, followed by a decrement of the number of active chips. Moving the
> decrement above the atomic_dec fixes the UAF.
> 
> [ The original problem was introduced in 3.1 kernel, while it was
>   developed in a different form.  The Fixes tag below indicates the
>   original commit but it doesn't mean that the patch is applicable
>   cleanly. -- tiwai ]
> 
> Fixes: 362e4e49abe5 ("ALSA: usb-audio - clear chip->probing on error exit")
> Reported-by: Hui Peng <benquike@gmail.com>
> Reported-by: Mathias Payer <mathias.payer@nebelwelt.net>
> Signed-off-by: Hui Peng <benquike@gmail.com>
> Signed-off-by: Mathias Payer <mathias.payer@nebelwelt.net>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> [surenb@google.com: resolve 3.18 differences]
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> Analysis for 3.18 codebase:
> snd_usb_audio_create() sets card->device_data = chip
> snd_usb_audio_probe() calls snd_card_free() and then resets chip->probing
> snd_card_free() results in the following call chain:
>  snd_card_free_when_closed() which waits on release_completion
>  snd_card_do_free() calls snd_device_free_all() and signals release_completion
>  snd_card_do_free() calls __snd_device_free()
>  __snd_device_free() calls dev->ops->dev_free() == snd_usb_audio_dev_free()
>  snd_usb_audio_dev_free() calls snd_usb_audio_free(chip) and frees "chip"
> chip->probing = 0 results in UAF

Thanks for the backport.  I've added it to my "internal" 3.18 queue.  As
there is not going to be any more public kernel.org 3.18.y releases,
I'll just be doing updates in the android-common tree over time every
few weeks.  This will end up in the next batch that gets merged into
there, thanks!

greg k-h