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

sound/usb/card.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/sound/usb/card.c b/sound/usb/card.c index f7dbdc10bf77..59fb1ef3cd55 100644 --- a/sound/usb/card.c +++ b/sound/usb/card.c @@ -593,9 +593,12 @@ snd_usb_audio_probe(struct usb_device *dev,

__error: if (chip) { + /* chip->probing is inside the chip->card object, + * reset before memory is possibly returned. + */ + chip->probing = 0; if (!chip->num_interfaces) snd_card_free(chip->card); - chip->probing = 0; } mutex_unlock(&register_mutex); __err_val: