Re: [PATCH v3 2/2] usb: gadget: uvc: prevent de-allocating inflight usb_requests

From: Michael Grzeschik
Date: Thu Sep 28 2023 - 17:04:24 EST

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In reply to: Avichal Rakesh: "[PATCH v3 2/2] usb: gadget: uvc: prevent de-allocating inflight usb_requests"
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Hi Avichal

Thanks for the patch!

On Thu, Sep 28, 2023 at 01:19:04PM -0700, Avichal Rakesh wrote:

Currently, when stopping the stream, uvcg_video_enable immediately
deallocates the usb_requests after calling usb_ep_dequeue. However,
usb_ep_dequeue is asynchronous and it is possible that it a request
that is still in use by the controller. This can lead to some hard
to reproduce crashes with double frees and general memory
inconsistencies.

This patch sets up some stronger guarantees around when a request should
be deallocated. To that extent, this patch does the following:
1. When stream is stopped only the currently owned uvc_requests are freed
and all in-flight uvc_requests are marked 'abandoned'
2. uvc_video_complete callback is made responsible for freeing up the
abandoned requests. No uvc specific logic is triggered when handling
abandoned requests.

This ensures that the ownership of uvc_request (and its corresponding
usb_request) is never ambiguous, and uvc_requests are always freed
regardless of when the requests are returned to the gadget driver.

Other changes in the patch are required to decouple the allocated
uvc_requests from uvc_video that allocated them.

Unfortunatly the format of this patch is relly heavy to review. You are
changing to much at once. Could you please finegrain your changes in
smaller overall topics.

Starting with the transition from the one big kcalloc to the chaned
list. This patch will be easy to review.

After that you could address this issue you are refering to.

This will help the review alot. I already saw some changes that are
probably even completely unrelated.

Link: https://lore.kernel.org/7cd81649-2795-45b6-8c10-b7df1055020d@xxxxxxxxxx
Suggested-by: Michael Grzeschik <mgr@xxxxxxxxxxxxxx>
Signed-off-by: Avichal Rakesh <arakesh@xxxxxxxxxx>
---
v1 -> v2: Switched from a timed wait to free on complete model based
on discussions with Michael.
v3 -> v3: Initialized ureqs at init to prevent accessing invalid
memory when no stream is created.

drivers/usb/gadget/function/uvc.h | 4 +-
drivers/usb/gadget/function/uvc_video.c | 183 +++++++++++++++++-------
2 files changed, 134 insertions(+), 53 deletions(-)

diff --git a/drivers/usb/gadget/function/uvc.h b/drivers/usb/gadget/function/uvc.h
index 989bc6b4e93d..e69cfb7cced1 100644
--- a/drivers/usb/gadget/function/uvc.h
+++ b/drivers/usb/gadget/function/uvc.h
@@ -81,6 +81,8 @@ struct uvc_request {
struct sg_table sgt;
u8 header[UVCG_REQUEST_HEADER_LEN];
struct uvc_buffer *last_buf;
+ struct list_head list;
+ bool is_abandoned;
};

struct uvc_video {
@@ -102,7 +104,7 @@ struct uvc_video {

/* Requests */
unsigned int req_size;
- struct uvc_request *ureq;
+ struct list_head ureqs; /* all uvc_requests allocated by uvc_video */
struct list_head req_free;
spinlock_t req_lock;

diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
index 70ff88854539..aa5ef63e5540 100644
--- a/drivers/usb/gadget/function/uvc_video.c
+++ b/drivers/usb/gadget/function/uvc_video.c
@@ -227,6 +227,23 @@ uvc_video_encode_isoc(struct usb_request *req, struct uvc_video *video,
* Request handling
*/

+static void uvc_video_free_request(struct uvc_request *ureq, struct usb_ep *ep)
+{
+ sg_free_table(&ureq->sgt);
+ if (ureq->req && ep) {
+ usb_ep_free_request(ep, ureq->req);
+ ureq->req = NULL;
+ }
+
+ kfree(ureq->req_buffer);
+ ureq->req_buffer = NULL;
+
+ if (!list_empty(&ureq->list))
+ list_del(&ureq->list);
+
+ kfree(ureq);
+}
+
static int uvcg_video_ep_queue(struct uvc_video *video, struct usb_request *req)
{
int ret;
@@ -254,7 +271,21 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req)
struct uvc_video *video = ureq->video;
struct uvc_video_queue *queue = &video->queue;
struct uvc_device *uvc = video->uvc;
+ struct uvc_buffer *last_buf;
unsigned long flags;
+ bool is_abandoned;
+
+ spin_lock_irqsave(&video->req_lock, flags);
+ is_abandoned = ureq->is_abandoned;
+ last_buf = ureq->last_buf;
+ ureq->last_buf = NULL;
+ spin_unlock_irqrestore(&video->req_lock, flags);
+
+ if (is_abandoned) {
+ uvcg_dbg(&video->uvc->func, "Freeing abandoned request\n");
+ uvc_video_free_request(ureq, ep);
+ return;
+ }

switch (req->status) {
case 0:
@@ -277,15 +308,27 @@ 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(&video->queue.irqlock, flags);
+ uvcg_complete_buffer(&video->queue, last_buf);
+ spin_unlock_irqrestore(&video->queue.irqlock, flags);
}

This change seems unrelated to the rest of the patch.

+ /*
+ * request might have been abandoned while being processed.
+ * do a last minute check before queueing the request back.
+ */
spin_lock_irqsave(&video->req_lock, flags);
- list_add_tail(&req->list, &video->req_free);
+ is_abandoned = ureq->is_abandoned;
+ if (!is_abandoned)
+ list_add_tail(&req->list, &video->req_free);
spin_unlock_irqrestore(&video->req_lock, flags);

+ if (is_abandoned) {
+ uvc_video_free_request(ureq, ep);
+ return;
+ }
+

This whole check of is_abondoned is really hard to read and understand.
Could it be somehow simplified?

if (uvc->state == UVC_STATE_STREAMING)
queue_work(video->async_wq, &video->pump);
}
@@ -293,25 +336,10 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req)
static int
uvc_video_free_requests(struct uvc_video *video)
{
- unsigned int i;
-
- if (video->ureq) {
- for (i = 0; i < video->uvc_num_requests; ++i) {
- sg_free_table(&video->ureq[i].sgt);
+ struct uvc_request *ureq, *temp;

- if (video->ureq[i].req) {
- usb_ep_free_request(video->ep, video->ureq[i].req);
- video->ureq[i].req = NULL;
- }
-
- if (video->ureq[i].req_buffer) {
- kfree(video->ureq[i].req_buffer);
- video->ureq[i].req_buffer = NULL;
- }
- }
-
- kfree(video->ureq);
- video->ureq = NULL;
+ list_for_each_entry_safe(ureq, temp, &video->ureqs, list) {
+ uvc_video_free_request(ureq, video->ep);

I thought we wanted to iterate over the req_free list in this function?

}

INIT_LIST_HEAD(&video->req_free);
@@ -322,6 +350,7 @@ uvc_video_free_requests(struct uvc_video *video)
static int
uvc_video_alloc_requests(struct uvc_video *video)
{
+ struct uvc_request *ureq;
unsigned int req_size;
unsigned int i;
int ret = -ENOMEM;
@@ -332,35 +361,36 @@ uvc_video_alloc_requests(struct uvc_video *video)
* max_t(unsigned int, video->ep->maxburst, 1)
* (video->ep->mult);

- video->ureq = kcalloc(video->uvc_num_requests, sizeof(struct uvc_request), GFP_KERNEL);
- if (video->ureq == NULL)
- return -ENOMEM;
-
- for (i = 0; i < video->uvc_num_requests; ++i) {
- video->ureq[i].req_buffer = kmalloc(req_size, GFP_KERNEL);
- if (video->ureq[i].req_buffer == NULL)
+ INIT_LIST_HEAD(&video->ureqs);
+ for (i = 0; i < video->uvc_num_requests; i++) {
+ ureq = kzalloc(sizeof(struct uvc_request), GFP_KERNEL);
+ if (ureq == NULL)
goto error;
+ INIT_LIST_HEAD(&ureq->list);
+ list_add_tail(&ureq->list, &video->ureqs);
+ }

- video->ureq[i].req = usb_ep_alloc_request(video->ep, GFP_KERNEL);
- if (video->ureq[i].req == NULL)
+ list_for_each_entry(ureq, &video->ureqs, list) {
+ ureq->req_buffer = kmalloc(req_size, GFP_KERNEL);
+ if (ureq->req_buffer == NULL)
goto error;
-
- video->ureq[i].req->buf = video->ureq[i].req_buffer;
- video->ureq[i].req->length = 0;
- video->ureq[i].req->complete = uvc_video_complete;
- video->ureq[i].req->context = &video->ureq[i];
- video->ureq[i].video = video;
- video->ureq[i].last_buf = NULL;
-
- list_add_tail(&video->ureq[i].req->list, &video->req_free);
+ ureq->req = usb_ep_alloc_request(video->ep, GFP_KERNEL);
+ if (ureq->req == NULL)
+ goto error;
+ ureq->req->buf = ureq->req_buffer;
+ ureq->req->length = 0;
+ ureq->req->complete = uvc_video_complete;
+ ureq->req->context = ureq;
+ ureq->video = video;
+ ureq->last_buf = NULL;
+ list_add_tail(&ureq->req->list, &video->req_free);
/* req_size/PAGE_SIZE + 1 for overruns and + 1 for header */
- sg_alloc_table(&video->ureq[i].sgt,
+ sg_alloc_table(&ureq->sgt,
DIV_ROUND_UP(req_size - UVCG_REQUEST_HEADER_LEN,
PAGE_SIZE) + 2, GFP_KERNEL);
}

Please move these changes to an separate patch.

video->req_size = req_size;
-
return 0;

error:
@@ -484,12 +514,68 @@ static void uvcg_video_pump(struct work_struct *work)
return;
}

+/*
+ * Disable video stream. This ensures that any inflight usb requests are marked
+ * for clean up and video buffers are dropped up before returning.
+ */
+static void uvcg_video_disable(struct uvc_video *video)
+{
+ struct uvc_buffer *buf, *tmp_buf;
+ struct uvc_request *ureq, *temp;
+ struct list_head buf_list; /* track in-flight video buffers */
+ struct usb_request *req;
+ unsigned long flags;
+
+ cancel_work_sync(&video->pump);
+ uvcg_queue_cancel(&video->queue, 0);
+
+ INIT_LIST_HEAD(&buf_list);
+ spin_lock_irqsave(&video->req_lock, flags);
+ /* abandon all usb requests */
+ list_for_each_entry_safe(ureq, temp, &video->ureqs, list) {
+ ureq->is_abandoned = true;
+ if (ureq->last_buf) {
+ list_add(&ureq->last_buf->queue, &buf_list);
+ ureq->last_buf = NULL;
+ }
+ list_del_init(&ureq->list);
+ if (ureq->req)
+ usb_ep_dequeue(video->ep, ureq->req);
+ }
+ /*
+ * re-add uvc_requests currently owned by the gadget to
+ * video->ureqs to be deallocated
+ */
+ list_for_each_entry(req, &video->req_free, list) {
+ ureq = req->context;
+ list_add_tail(&ureq->list, &video->ureqs);
+ }

So what you actually do is moving the requests from the free list
to the ureqs list. Would it not be possible to separate the cleanup
on two locations instead?

+ spin_unlock_irqrestore(&video->req_lock, flags);
+
+ /*
+ * drop abandoned uvc_buffers, as the completion handler
+ * no longer will
+ */
+ if (!list_empty(&buf_list)) {
+ spin_lock_irqsave(&video->queue.irqlock, flags);
+ list_for_each_entry_safe(buf, tmp_buf,
+ &buf_list, queue) {
+ video->queue.flags |= UVC_QUEUE_DROP_INCOMPLETE;
+ uvcg_complete_buffer(&video->queue, buf);
+ list_del(&buf->queue);
+ }
+ spin_unlock_irqrestore(&video->queue.irqlock, flags);
+ }
+
+ uvc_video_free_requests(video);
+ uvcg_queue_enable(&video->queue, 0);
+}
+
/*
* Enable or disable the video stream.
*/
int uvcg_video_enable(struct uvc_video *video, int enable)
{
- unsigned int i;
int ret;

if (video->ep == NULL) {
@@ -499,15 +585,7 @@ int uvcg_video_enable(struct uvc_video *video, int enable)
}

if (!enable) {
- cancel_work_sync(&video->pump);
- uvcg_queue_cancel(&video->queue, 0);
-
- for (i = 0; i < video->uvc_num_requests; ++i)
- if (video->ureq && video->ureq[i].req)
- usb_ep_dequeue(video->ep, video->ureq[i].req);
-
- uvc_video_free_requests(video);
- uvcg_queue_enable(&video->queue, 0);
+ uvcg_video_disable(video);
return 0;
}

@@ -536,6 +614,7 @@ int uvcg_video_enable(struct uvc_video *video, int enable)
*/
int uvcg_video_init(struct uvc_video *video, struct uvc_device *uvc)
{
+ INIT_LIST_HEAD(&video->ureqs);
INIT_LIST_HEAD(&video->req_free);
spin_lock_init(&video->req_lock);
INIT_WORK(&video->pump, uvcg_video_pump);
--
2.42.0.582.g8ccd20d70d-goog

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Next message: Darrick J. Wong: "Re: BUG: MADV_COLLAPSE doesn't work for XFS files]"
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In reply to: Avichal Rakesh: "[PATCH v3 2/2] usb: gadget: uvc: prevent de-allocating inflight usb_requests"
Next in thread: Avichal Rakesh: "Re: [PATCH v3 2/2] usb: gadget: uvc: prevent de-allocating inflight usb_requests"
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