FAILED: patch "[PATCH] usb: gadget: f_fs: Fix race between aio_cancel() and AIO" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 24729b307eefcd7c476065cd7351c1a018082c19
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024051302-basin-buffed-b0cd@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

24729b307eef ("usb: gadget: f_fs: Fix race between aio_cancel() and AIO request complete") b566d38857fc ("usb: gadget: f_fs: use io_data->status consistently")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 24729b307eefcd7c476065cd7351c1a018082c19 Mon Sep 17 00:00:00 2001
From: Wesley Cheng <[email protected]>
Date: Mon, 8 Apr 2024 18:40:59 -0700
Subject: [PATCH] usb: gadget: f_fs: Fix race between aio_cancel() and AIO
request complete

FFS based applications can utilize the aio_cancel() callback to dequeue pending USB requests submitted to the UDC. There is a scenario where the FFS application issues an AIO cancel call, while the UDC is handling a soft disconnect. For a DWC3 based implementation, the callstack looks like the following:

DWC3 Gadget FFS Application
dwc3_gadget_soft_disconnect() ...
--> dwc3_stop_active_transfers()

    --> dwc3_gadget_giveback(-ESHUTDOWN)
      --> ffs_epfile_async_io_complete()   ffs_aio_cancel()
        --> usb_ep_free_request()            --> usb_ep_dequeue()

There is currently no locking implemented between the AIO completion handler and AIO cancel, so the issue occurs if the completion routine is running in parallel to an AIO cancel call coming from the FFS application. As the completion call frees the USB request (io_data->req) the FFS application is also referencing it for the usb_ep_dequeue() call. This can lead to accessing a stale/hanging pointer.

commit b566d38857fc ("usb: gadget: f_fs: use io_data->status consistently") relocated the usb_ep_free_request() into ffs_epfile_async_io_complete(). However, in order to properly implement locking to mitigate this issue, the spinlock can't be added to ffs_epfile_async_io_complete(), as usb_ep_dequeue() (if successfully dequeuing a USB request) will call the function driver's completion handler in the same context. Hence, leading into a deadlock.

Fix this issue by moving the usb_ep_free_request() back to ffs_user_copy_worker(), and ensuring that it explicitly sets io_data->req to NULL after freeing it within the ffs->eps_lock. This resolves the race condition above, as the ffs_aio_cancel() routine will not continue attempting to dequeue a request that has already been freed, or the ffs_user_copy_work() not freeing the USB request until the AIO cancel is done referencing it.

This fix depends on
commit b566d38857fc ("usb: gadget: f_fs: use io_data->status consistently")

index f855f1fc8e5e..aa80c2a6b8e0 100644
@@ -852,6 +852,7 @@ static void ffs_user_copy_worker(struct work_struct *work)
 						   work);
 	int ret = io_data->status;
 	bool kiocb_has_eventfd = io_data->kiocb->ki_flags & IOCB_EVENTFD;
+	unsigned long flags;
 
 	if (io_data->read && ret > 0) {
 		kthread_use_mm(io_data->mm);

@@ -864,6 +865,11 @@ static void ffs_user_copy_worker(struct work_struct *work)
 	if (io_data->ffs->ffs_eventfd && !kiocb_has_eventfd)
 		eventfd_signal(io_data->ffs->ffs_eventfd);
 
+	spin_lock_irqsave(&io_data->ffs->eps_lock, flags);
+	usb_ep_free_request(io_data->ep, io_data->req);
+	io_data->req = NULL;
+	spin_unlock_irqrestore(&io_data->ffs->eps_lock, flags);
+
 	if (io_data->read)
 		kfree(io_data->to_free);
 	ffs_free_buffer(io_data);

@@ -877,7 +883,6 @@ static void ffs_epfile_async_io_complete(struct usb_ep *_ep,
 	struct ffs_data *ffs = io_data->ffs;
 
 	io_data->status = req->status ? req->status : req->actual;
-	usb_ep_free_request(_ep, req);
 
 	INIT_WORK(&io_data->work, ffs_user_copy_worker);
 	queue_work(ffs->io_completion_wq, &io_data->work);