usb: musb: Fix hardware lockup on first Rx endpoint request

In the Linux kernel, the following vulnerability has been resolved:

usb: musb: Fix hardware lockup on first Rx endpoint request

There is a possibility that a request's callback could be invoked from

usb_ep_queue() (call trace below, supplemented with missing calls):

req->complete from usb_gadget_giveback_request

(drivers/usb/gadget/udc/core.c:999)

usb_gadget_giveback_request from musb_g_giveback

(drivers/usb/musb/musb_gadget.c:147)

musb_g_giveback from rxstate

(drivers/usb/musb/musb_gadget.c:784)

rxstate from musb_ep_restart

(drivers/usb/musb/musb_gadget.c:1169)

musb_ep_restart from musb_ep_restart_resume_work

(drivers/usb/musb/musb_gadget.c:1176)

musb_ep_restart_resume_work from musb_queue_resume_work

(drivers/usb/musb/musb_core.c:2279)

musb_queue_resume_work from musb_gadget_queue

(drivers/usb/musb/musb_gadget.c:1241)

musb_gadget_queue from usb_ep_queue

(drivers/usb/gadget/udc/core.c:300)

According to the docstring of usb_ep_queue(), this should not happen:

"Note that @req's ->complete() callback must never be called from within

usb_ep_queue() as that can create deadlock situations."

In fact, a hardware lockup might occur in the following sequence:

1. The gadget is initialized using musb_gadget_enable().

2. Meanwhile, a packet arrives, and the RXPKTRDY flag is set, raising an

interrupt.

3. If IRQs are enabled, the interrupt is handled, but musb_g_rx() finds an

empty queue (next_request() returns NULL). The interrupt flag has

already been cleared by the glue layer handler, but the RXPKTRDY flag

remains set.

4. The first request is enqueued using usb_ep_queue(), leading to the call

of req->complete(), as shown in the call trace above.

5. If the callback enables IRQs and another packet is waiting, step (3)

repeats. The request queue is empty because usb_g_giveback() removes the

request before invoking the callback.

6. The endpoint remains locked up, as the interrupt triggered by hardware

setting the RXPKTRDY flag has been handled, but the flag itself remains

set.

For this scenario to occur, it is only necessary for IRQs to be enabled at

some point during the complete callback. This happens with the USB Ethernet

gadget, whose rx_complete() callback calls netif_rx(). If called in the

task context, netif_rx() disables the bottom halves (BHs). When the BHs are

re-enabled, IRQs are also enabled to allow soft IRQs to be processed. The

gadget itself is initialized at module load (or at boot if built-in), but

the first request is enqueued when the network interface is brought up,

triggering rx_complete() in the task context via ioctl(). If a packet

arrives while the interface is down, it can prevent the interface from

receiving any further packets from the USB host.

The situation is quite complicated with many parties involved. This

particular issue can be resolved in several possible ways:

1. Ensure that callbacks never enable IRQs. This would be difficult to

enforce, as discovering how netif_rx() interacts with interrupts was

already quite challenging and u_ether is not the only function driver.

Similar "bugs" could be hidden in other drivers as well.

2. Disable MUSB interrupts in musb_g_giveback() before calling the callback

and re-enable them afterwars (by calling musb_{dis,en}able_interrupts(),

for example). This would ensure that MUSB interrupts are not handled

during the callback, even if IRQs are enabled. In fact, it would allow

IRQs to be enabled when releasing the lock. However, this feels like an

inelegant hack.

3. Modify the interrupt handler to clear the RXPKTRDY flag if the request

queue is empty. While this approach also feels like a hack, it wastes

CPU time by attempting to handle incoming packets when the software is

not ready to process them.

4. Flush the Rx FIFO instead of calling rxstate() in musb_ep_restart().

This ensures that the hardware can receive packets when there is at

least one request in the queue. Once I

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