Re: [PATCH v9 4/4] usb: gadget: uvc: Fix use-after-free for inflight usb_requests

From: Dan Scally
Date: Sat Oct 28 2023 - 16:57:04 EST

Hi Avichal

On 27/10/2023 21:19, Avichal Rakesh wrote:
Currently, the uvc gadget driver allocates all uvc_requests as one array
and deallocates them all when the video stream stops. This includes
de-allocating all the usb_requests associated with those uvc_requests.
This can lead to use-after-free issues if any of those de-allocated
usb_requests were still owned by the usb controller.

This is patch 2 of 2 in fixing the use-after-free issue. It adds a new
flag to uvc_video to track when frames and requests should be flowing.
When disabling the video stream, the flag is tripped and, instead
of de-allocating all uvc_requests and usb_requests, the gadget
driver only de-allocates those usb_requests that are currently
owned by it (as present in req_free). Other usb_requests are left
untouched until their completion handler is called which takes care
of freeing the usb_request and its corresponding uvc_request.

Now that uvc_video does not depends on uvc->state, this patch removes
unnecessary upates to uvc->state that were made to accommodate uvc_video
logic. This should ensure that uvc gadget driver never accidentally
de-allocates a usb_request that it doesn't own.

Link: https://lore.kernel.org/7cd81649-2795-45b6-8c10-b7df1055020d@xxxxxxxxxx
Suggested-by: Michael Grzeschik <m.grzeschik@xxxxxxxxxxxxxx>
Reviewed-by: Michael Grzeschik <m.grzeschik@xxxxxxxxxxxxxx>
Tested-by: Michael Grzeschik <m.grzeschik@xxxxxxxxxxxxxx>
Signed-off-by: Avichal Rakesh <arakesh@xxxxxxxxxx>
---
v1 -> v2: Rebased to ToT, and fixed deadlock reported in
https://lore.kernel.org/all/ZRv2UnKztgyqk2pt@xxxxxxxxxxxxxx/
v2 -> v3: Fix email threading goof-up
v3 -> v4: re-rebase to ToT & moved to a uvc_video level lock
as discussed in
https://lore.kernel.org/b14b296f-2e08-4edf-aeea-1c5b621e2d0c@xxxxxxxxxx/
v4 -> v5: Address review comments. Add Reviewed-by & Tested-by.
v5 -> v6: Added another patch before this one to make uvcg_video_disable
easier to review.
v6 -> v7: Fix warning reported in
https://lore.kernel.org/202310200457.GwPPFuHX-lkp@xxxxxxxxx/
v7 -> v8: No change. Getting back in review queue
v8 -> v9: No change.

drivers/usb/gadget/function/uvc.h | 1 +
drivers/usb/gadget/function/uvc_v4l2.c | 12 +--
drivers/usb/gadget/function/uvc_video.c | 128 ++++++++++++++++++++----
3 files changed, 111 insertions(+), 30 deletions(-)

diff --git a/drivers/usb/gadget/function/uvc.h b/drivers/usb/gadget/function/uvc.h
index 993694da0bbc..be0d012aa244 100644
--- a/drivers/usb/gadget/function/uvc.h
+++ b/drivers/usb/gadget/function/uvc.h
@@ -102,6 +102,7 @@ struct uvc_video {
unsigned int uvc_num_requests;

/* Requests */
+ bool is_enabled; /* tracks whether video stream is enabled */
unsigned int req_size;
struct list_head ureqs; /* all uvc_requests allocated by uvc_video */
struct list_head req_free;
diff --git a/drivers/usb/gadget/function/uvc_v4l2.c b/drivers/usb/gadget/function/uvc_v4l2.c
index 904dd283cbf7..2f8634e05612 100644
--- a/drivers/usb/gadget/function/uvc_v4l2.c
+++ b/drivers/usb/gadget/function/uvc_v4l2.c
@@ -451,8 +451,8 @@ uvc_v4l2_streamon(struct file *file, void *fh, enum v4l2_buf_type type)
* Complete the alternate setting selection setup phase now that
* userspace is ready to provide video frames.
*/
- uvc_function_setup_continue(uvc, 0);
uvc->state = UVC_STATE_STREAMING;
+ uvc_function_setup_continue(uvc, 0);

return 0;
}
@@ -468,11 +468,11 @@ uvc_v4l2_streamoff(struct file *file, void *fh, enum v4l2_buf_type type)
if (type != video->queue.queue.type)
return -EINVAL;

- uvc->state = UVC_STATE_CONNECTED;
ret = uvcg_video_disable(video);
if (ret < 0)
return ret;

+ uvc->state = UVC_STATE_CONNECTED;
uvc_function_setup_continue(uvc, 1);
return 0;
}


I'm not sure I understand what these re-orderings are for...can you explain please?

@@ -507,14 +507,6 @@ uvc_v4l2_subscribe_event(struct v4l2_fh *fh,
static void uvc_v4l2_disable(struct uvc_device *uvc)
{
uvc_function_disconnect(uvc);
- /*
- * Drop uvc->state to CONNECTED if it was streaming before.
- * This ensures that the usb_requests are no longer queued
- * to the controller.
- */
- if (uvc->state == UVC_STATE_STREAMING)
- uvc->state = UVC_STATE_CONNECTED;
-
uvcg_video_disable(&uvc->video);
uvcg_free_buffers(&uvc->video.queue);
uvc->func_connected = false;
diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
index 1081dd790fd6..8f330ce696ec 100644
--- a/drivers/usb/gadget/function/uvc_video.c
+++ b/drivers/usb/gadget/function/uvc_video.c
@@ -227,6 +227,9 @@ uvc_video_encode_isoc(struct usb_request *req, struct uvc_video *video,
* Request handling
*/

+/*
+ * Must be called with req_lock held as it modifies the list ureq is held in
+ */



This comment probably belongs in patch #2. And in that case, shouldn't uvc_video_free_requests() hold the lock in that patch?

static void
uvc_video_free_request(struct uvc_request *ureq, struct usb_ep *ep)
{
@@ -271,9 +274,25 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req)
struct uvc_request *ureq = req->context;
struct uvc_video *video = ureq->video;
struct uvc_video_queue *queue = &video->queue;
- struct uvc_device *uvc = video->uvc;
+ struct uvc_buffer *last_buf = NULL;
unsigned long flags;

+ spin_lock_irqsave(&video->req_lock, flags);
+ if (!video->is_enabled) {
+ /*
+ * When is_enabled is false, uvc_video_disable ensures that
s/uvc_video_disable/uvc_video_disable()
+ * in-flight uvc_buffers are returned, so we can safely
+ * call free_request without worrying about last_buf.
+ */
+ uvc_video_free_request(ureq, ep);
Now I understand the conditional in this function in patch 2 :)
+ spin_unlock_irqrestore(&video->req_lock, flags);
+ return;
+ }
+
+ last_buf = ureq->last_buf;
+ ureq->last_buf = NULL;
+ spin_unlock_irqrestore(&video->req_lock, flags);


I'm not a huge fan of this locking, unlocking and relocking the same spinlock within the same function. Can we just hold the lock for the duration? if not, can there be an explanatory comment as to why?
+
switch (req->status) {
case 0:
break;
@@ -295,17 +314,26 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req)
uvcg_queue_cancel(queue, 0);
}

- if (ureq->last_buf) {
- uvcg_complete_buffer(&video->queue, ureq->last_buf);
- ureq->last_buf = NULL;
+ if (last_buf) {
+ spin_lock_irqsave(&queue->irqlock, flags);
+ uvcg_complete_buffer(&video->queue, last_buf);
+ spin_unlock_irqrestore(&queue->irqlock, flags);



I think it's right to take the irqlock here but it probably should have always been held, so this probably ought to go in its own commit with a Fixes:

}

spin_lock_irqsave(&video->req_lock, flags);
- list_add_tail(&req->list, &video->req_free);
- spin_unlock_irqrestore(&video->req_lock, flags);
-
- if (uvc->state == UVC_STATE_STREAMING)
+ /*
+ * Video stream might have been disabled while we were
+ * processing the current usb_request. So make sure
+ * we're still streaming before queueing the usb_request
+ * back to req_free
+ */
+ if (video->is_enabled) {
+ list_add_tail(&req->list, &video->req_free);
queue_work(video->async_wq, &video->pump);
+ } else {
+ uvc_video_free_request(ureq, ep);
+ }
+ spin_unlock_irqrestore(&video->req_lock, flags);
}

static int
@@ -393,20 +421,22 @@ static void uvcg_video_pump(struct work_struct *work)
struct uvc_video_queue *queue = &video->queue;
/* video->max_payload_size is only set when using bulk transfer */
bool is_bulk = video->max_payload_size;
- struct uvc_device *uvc = video->uvc;
struct usb_request *req = NULL;
struct uvc_buffer *buf;
unsigned long flags;
bool buf_done;
int ret;

- while (uvc->state == UVC_STATE_STREAMING && video->ep->enabled) {
+ while (true) {
+ if (!video->ep->enabled)
+ return;
+
/*
- * Retrieve the first available USB request, protected by the
- * request lock.
+ * Check is_enabled and retrieve the first available USB
+ * request, protected by the request lock.
*/
spin_lock_irqsave(&video->req_lock, flags);
- if (list_empty(&video->req_free)) {
+ if (!video->is_enabled || list_empty(&video->req_free)) {
spin_unlock_irqrestore(&video->req_lock, flags);
return;
}
@@ -488,9 +518,11 @@ static void uvcg_video_pump(struct work_struct *work)
return;

spin_lock_irqsave(&video->req_lock, flags);
- list_add_tail(&req->list, &video->req_free);
+ if (video->is_enabled)
+ list_add_tail(&req->list, &video->req_free);
+ else
+ uvc_video_free_request(req->context, video->ep);
spin_unlock_irqrestore(&video->req_lock, flags);
- return;
}

/*
@@ -499,7 +531,11 @@ static void uvcg_video_pump(struct work_struct *work)
int
uvcg_video_disable(struct uvc_video *video)
{
- struct uvc_request *ureq;
+ unsigned long flags;
+ struct list_head inflight_bufs;
+ struct usb_request *req, *temp;
+ struct uvc_buffer *buf, *btemp;
+ struct uvc_request *ureq, *utemp;

if (video->ep == NULL) {
uvcg_info(&video->uvc->func,
@@ -507,15 +543,58 @@ uvcg_video_disable(struct uvc_video *video)
return -ENODEV;
}

+ INIT_LIST_HEAD(&inflight_bufs);
+ spin_lock_irqsave(&video->req_lock, flags);
+ video->is_enabled = false;
+
+ /*
+ * Remove any in-flight buffers from the uvc_requests
+ * because we want to return them before cancelling the
+ * queue. This ensures that we aren't stuck waiting for
+ * all complete callbacks to come through before disabling
+ * vb2 queue.
+ */
+ list_for_each_entry(ureq, &video->ureqs, list) {
+ if (ureq->last_buf) {
+ list_add_tail(&ureq->last_buf->queue, &inflight_bufs);
+ ureq->last_buf = NULL;
+ }
+ }
+ spin_unlock_irqrestore(&video->req_lock, flags);
+
cancel_work_sync(&video->pump);
uvcg_queue_cancel(&video->queue, 0);

- list_for_each_entry(ureq, &video->ureqs, list) {
- if (ureq->req)
- usb_ep_dequeue(video->ep, ureq->req);
+ spin_lock_irqsave(&video->req_lock, flags);
+ /*
+ * Remove all uvc_reqeusts from ureqs with list_del_init
+ * This lets uvc_video_free_request correctly identify
+ * if the uvc_request is attached to a list or not when freeing
+ * memory.
+ */
+ list_for_each_entry_safe(ureq, utemp, &video->ureqs, list)
+ list_del_init(&ureq->list);
+
+ list_for_each_entry_safe(req, temp, &video->req_free, list) {
+ list_del(&req->list);
+ uvc_video_free_request(req->context, video->ep);
}

- uvc_video_free_requests(video);
+ INIT_LIST_HEAD(&video->ureqs);
+ INIT_LIST_HEAD(&video->req_free);
+ video->req_size = 0;
+ spin_unlock_irqrestore(&video->req_lock, flags);
+
+ /*
+ * Return all the video buffers before disabling the queue.
+ */
+ spin_lock_irqsave(&video->queue.irqlock, flags);
+ list_for_each_entry_safe(buf, btemp, &inflight_bufs, queue) {
+ list_del(&buf->queue);
+ uvcg_complete_buffer(&video->queue, buf);
+ }
+ spin_unlock_irqrestore(&video->queue.irqlock, flags);
+
uvcg_queue_enable(&video->queue, 0);
return 0;
}
@@ -533,6 +612,14 @@ int uvcg_video_enable(struct uvc_video *video)
return -ENODEV;
}

+ /*
+ * Safe to access request related fields without req_lock because
+ * this is the only thread currently active, and no other
+ * request handling thread will become active until this function
+ * returns.
+ */
+ video->is_enabled = true;
+
if ((ret = uvcg_queue_enable(&video->queue, 1)) < 0)
return ret;

@@ -558,6 +645,7 @@ int uvcg_video_enable(struct uvc_video *video)
*/
int uvcg_video_init(struct uvc_video *video, struct uvc_device *uvc)
{
+ video->is_enabled = false;
INIT_LIST_HEAD(&video->ureqs);
INIT_LIST_HEAD(&video->req_free);
spin_lock_init(&video->req_lock);
--
2.42.0.820.g83a721a137-goog