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 stable@vger.kernel.org.

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 'stable@vger.kernel.org' --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 stern@rowland.harvard.edu 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.

Signed-off-by: Alan Stern stern@rowland.harvard.edu Cc: stable stable@kernel.org Fixes: b4dbda1a22d2 ("USB: dummy-hcd: disable interrupts during req->complete") Reported-by: Yunseong Kim ysk@kzalloc.com Closes: https://lore.kernel.org/linux-usb/5b337389-73b9-4ee4-a83e-7e82bf5af87a@kzalloc.com/ Reported-by: syzbot+8baacc4139f12fa77909@syzkaller.appspotmail.com Closes: https://lore.kernel.org/linux-usb/68ac2411.050a0220.37038e.0087.GAE@google.com/ Tested-by: syzbot+8baacc4139f12fa77909@syzkaller.appspotmail.com CC: Sebastian Andrzej Siewior bigeasy@linutronix.de CC: stable@vger.kernel.org Reviewed-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://lore.kernel.org/r/bb192ae2-4eee-48ee-981f-3efdbbd0d8f0@rowland.harva... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c index 21dbfb0b3bac..1cefca660773 100644 --- a/drivers/usb/gadget/udc/dummy_hcd.c +++ b/drivers/usb/gadget/udc/dummy_hcd.c @@ -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; }