Message-ID: <2024122834-CVE-2024-56687-99e2@gregkh>
Date: Sat, 28 Dec 2024 10:45:39 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-56687: usb: musb: Fix hardware lockup on first Rx endpoint request

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."

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 IRQs are enabled, the interrupt
   handler will be able to correctly process the next incoming packet
   (eventually calling rxstate()). This approach may cause one or two
   packets to be dropped (two if double buffering is enabled), but this
   seems to be a minor issue, as packet loss can occur when the software is
   not yet ready to process them. Additionally, this solution makes the
   gadget driver compliant with the rule mentioned in the docstring of
   usb_ep_queue().

There may be additional solutions, but from these four, the last one has
been chosen as it seems to be the most appropriate, as it addresses the
"bad" behavior of the driver.

The Linux kernel CVE team has assigned CVE-2024-56687 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.18 with commit baebdf48c360080710f80699eea3affbb13d6c65 and fixed in 6.1.120 with commit c749500b28cae67410792096133ee7f282439c51
	Issue introduced in 5.18 with commit baebdf48c360080710f80699eea3affbb13d6c65 and fixed in 6.6.64 with commit 5906ee3693674d734177df13a519a21bb03f730d
	Issue introduced in 5.18 with commit baebdf48c360080710f80699eea3affbb13d6c65 and fixed in 6.11.11 with commit f05ad9755bb294328c3d0f429164ac6d4d08c548
	Issue introduced in 5.18 with commit baebdf48c360080710f80699eea3affbb13d6c65 and fixed in 6.12.2 with commit 0c89445e6d475b78d37b64ae520831cd43af7db4
	Issue introduced in 5.18 with commit baebdf48c360080710f80699eea3affbb13d6c65 and fixed in 6.13-rc1 with commit 3fc137386c4620305bbc2a216868c53f9245670a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-56687
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/usb/musb/musb_gadget.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/c749500b28cae67410792096133ee7f282439c51
	https://git.kernel.org/stable/c/5906ee3693674d734177df13a519a21bb03f730d
	https://git.kernel.org/stable/c/f05ad9755bb294328c3d0f429164ac6d4d08c548
	https://git.kernel.org/stable/c/0c89445e6d475b78d37b64ae520831cd43af7db4
	https://git.kernel.org/stable/c/3fc137386c4620305bbc2a216868c53f9245670a

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