FAILED: patch "[PATCH] USB: gadget: dummy-hcd: Fix locking bug in RT-enabled kernels" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-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-5.4.y
git checkout FETCH_HEAD
git cherry-pick -x 8d63c83d8eb922f6c316320f50c82fa88d099bea
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025091724-yeast-sublet-dc69@gregkh' --subject-prefix 'PATCH 5.4.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 8d63c83d8eb922f6c316320f50c82fa88d099bea Mon Sep 17 00:00:00 2001
From: Alan Stern <[email protected]>
Date: Mon, 25 Aug 2025 12:00:22 -0400
Subject: [PATCH] USB: gadget: dummy-hcd: Fix locking bug in RT-enabled kernels

Yunseong Kim and the syzbot fuzzer both reported a problem in RT-enabled kernels caused by the way dummy-hcd mixes interrupt management and spin-locking. The pattern was:

	local_irq_save(flags);
	spin_lock(&dum->lock);
	...
	spin_unlock(&dum->lock);
	...		// calls usb_gadget_giveback_request()
	local_irq_restore(flags);

The code was written this way because usb_gadget_giveback_request() needs to be called with interrupts disabled and the private lock not held.

While this pattern works fine in non-RT kernels, it's not good when RT is enabled. RT kernels handle spinlocks much like mutexes; in particular, spin_lock() may sleep. But sleeping is not allowed while local interrupts are disabled.

To fix the problem, rewrite the code to conform to the pattern used elsewhere in dummy-hcd and other UDC drivers:

	spin_lock_irqsave(&dum->lock, flags);
	...
	spin_unlock(&dum->lock);
	usb_gadget_giveback_request(...);
	spin_lock(&dum->lock);
	...
	spin_unlock_irqrestore(&dum->lock, flags);

This approach satisfies the RT requirements.

index 21dbfb0b3bac..1cefca660773 100644
@@ -765,8 +765,7 @@ static int dummy_dequeue(struct usb_ep *_ep, struct usb_request *_req)
 	if (!dum->driver)
 		return -ESHUTDOWN;
 
-	local_irq_save(flags);
-	spin_lock(&dum->lock);
+	spin_lock_irqsave(&dum->lock, flags);
 	list_for_each_entry(iter, &ep->queue, queue) {
 		if (&iter->req != _req)
 			continue;

@@ -776,15 +775,16 @@ static int dummy_dequeue(struct usb_ep *_ep, struct usb_request *_req)
 		retval = 0;
 		break;
 	}
-	spin_unlock(&dum->lock);
 
 	if (retval == 0) {
 		dev_dbg(udc_dev(dum),
 				"dequeued req %p from %s, len %d buf %p\n",
 				req, _ep->name, _req->length, _req->buf);
+		spin_unlock(&dum->lock);
 		usb_gadget_giveback_request(_ep, _req);
+		spin_lock(&dum->lock);
 	}
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&dum->lock, flags);
 	return retval;
 }
 

From: Jakob Koschel <[email protected]>

[ Upstream commit 7975f080d3557725160a878b1a64339043ba3d91 ]

To move the list iterator variable into the list_for_each_entry_*() macro in the future it should be avoided to use the list iterator variable after the loop body.

To *never* use the list iterator variable after the loop it was concluded to use a separate iterator variable [1].

Stable-dep-of: 8d63c83d8eb9 ("USB: gadget: dummy-hcd: Fix locking bug in RT-enabled kernels")

---
 drivers/usb/gadget/udc/dummy_hcd.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

index 58261ec1300a8..47e97679a2adf 100644
@@ -748,7 +748,7 @@ static int dummy_dequeue(struct usb_ep *_ep, struct usb_request *_req)
 	struct dummy		*dum;
 	int			retval = -EINVAL;
 	unsigned long		flags;
-	struct dummy_request	*req = NULL;
+	struct dummy_request	*req = NULL, *iter;
 
 	if (!_ep || !_req)
 		return retval;

@@ -760,13 +760,14 @@ static int dummy_dequeue(struct usb_ep *_ep, struct usb_request *_req)
 
 	local_irq_save(flags);
 	spin_lock(&dum->lock);
-	list_for_each_entry(req, &ep->queue, queue) {
-		if (&req->req == _req) {
-			list_del_init(&req->queue);
-			_req->status = -ECONNRESET;
-			retval = 0;
-			break;
-		}
+	list_for_each_entry(iter, &ep->queue, queue) {
+		if (&iter->req != _req)
+			continue;
+		list_del_init(&iter->queue);
+		_req->status = -ECONNRESET;
+		req = iter;
+		retval = 0;
+		break;
 	}
 	spin_unlock(&dum->lock);
 

2.51.0

From: Alan Stern <[email protected]>

[ Upstream commit 8d63c83d8eb922f6c316320f50c82fa88d099bea ]

Yunseong Kim and the syzbot fuzzer both reported a problem in RT-enabled kernels caused by the way dummy-hcd mixes interrupt management and spin-locking. The pattern was:

	local_irq_save(flags);
	spin_lock(&dum->lock);
	...
	spin_unlock(&dum->lock);
	...		// calls usb_gadget_giveback_request()
	local_irq_restore(flags);

The code was written this way because usb_gadget_giveback_request() needs to be called with interrupts disabled and the private lock not held.

While this pattern works fine in non-RT kernels, it's not good when RT is enabled. RT kernels handle spinlocks much like mutexes; in particular, spin_lock() may sleep. But sleeping is not allowed while local interrupts are disabled.

To fix the problem, rewrite the code to conform to the pattern used elsewhere in dummy-hcd and other UDC drivers:

	spin_lock_irqsave(&dum->lock, flags);
	...
	spin_unlock(&dum->lock);
	usb_gadget_giveback_request(...);
	spin_lock(&dum->lock);
	...
	spin_unlock_irqrestore(&dum->lock, flags);

This approach satisfies the RT requirements.

---
 drivers/usb/gadget/udc/dummy_hcd.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index 47e97679a2adf..7790a1a9c9330 100644
@@ -758,8 +758,7 @@ static int dummy_dequeue(struct usb_ep *_ep, struct usb_request *_req)
 	if (!dum->driver)
 		return -ESHUTDOWN;
 
-	local_irq_save(flags);
-	spin_lock(&dum->lock);
+	spin_lock_irqsave(&dum->lock, flags);
 	list_for_each_entry(iter, &ep->queue, queue) {
 		if (&iter->req != _req)
 			continue;

@@ -769,15 +768,16 @@ static int dummy_dequeue(struct usb_ep *_ep, struct usb_request *_req)
 		retval = 0;
 		break;
 	}
-	spin_unlock(&dum->lock);
 
 	if (retval == 0) {
 		dev_dbg(udc_dev(dum),
 				"dequeued req %p from %s, len %d buf %p\n",
 				req, _ep->name, _req->length, _req->buf);
+		spin_unlock(&dum->lock);
 		usb_gadget_giveback_request(_ep, _req);
+		spin_lock(&dum->lock);
 	}
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&dum->lock, flags);
 	return retval;
 }
 

2.51.0