CVE-2024-56687 Information

Dec 30, 2024 cve

Description

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.\n 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 ugs\ 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_disenable_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

truncated—

Reference

https://git.kernel.org/stable/c/0c89445e6d475b78d37b64ae520831cd43af7db4 https://git.kernel.org/stable/c/3fc137386c4620305bbc2a216868c53f9245670a https://git.kernel.org/stable/c/5906ee3693674d734177df13a519a21bb03f730d https://git.kernel.org/stable/c/c749500b28cae67410792096133ee7f282439c51 https://git.kernel.org/stable/c/f05ad9755bb294328c3d0f429164ac6d4d08c548

Share on: