CVE-2026-89732: usb: gadget: f_fs: Prevent deadlock during ep0 read loop

CVE-2026-89732: usb: gadget: f_fs: Prevent deadlock during ep0 read loop

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

usb: gadget: f_fs: Prevent deadlock during ep0 read loop

Currently, ffs_ep0_read() holds ffs->mutex when it prepares to go to
sleep waiting for an event. When no setup events are pending, it calls
wait_event_interruptible_exclusive_locked_irq() with the mutex still
held. The wait macro deliberately drops the waitqueue spinlock before
sleeping but does not drop the mutex.

If a userspace daemon is polling ep0 via read() and the gadget is
asynchronously torn down via configfs (e.g., echo "" > UDC), a
deadlock can occur:

1. The configfs teardown calls functionfs_unbind(), which queues a
   FUNCTIONFS_UNBIND event.
2. The daemon wakes up, consumes the event, and drops the mutex.
3. However, if the daemon loops and immediately issues another read()
   before exiting, it reacquires ffs->mutex and again goes into an
   interruptible sleep.
4. Meanwhile, functionfs_unbind() continues execution and attempts to
   acquire ffs->mutex to tear down ep0req.
5. The kernel deadlocks because the configfs thread is stuck in an
   uninterruptible sleep waiting for the mutex, while the userspace
   daemon is in an interruptible sleep holding the mutex forever
   because no more events will arrive.

To fix this, we drop both the waitqueue spinlock and ffs->mutex before
going to sleep, and use wait_event_interruptible_exclusive() instead.
Upon waking up, we jump back to the `retry` label to safely reacquire
the mutex and re-evaluate the state machine. By not sleeping with
ffs->mutex held, we natively decouple gadget teardowns (which require
the mutex) from userspace polling.

The Linux kernel CVE team has assigned CVE-2026-89732 to this issue.


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

	Issue introduced in 2.6.35 with commit ddf8abd2599491cbad959c700b90ba72a5dce8d0 and fixed in 6.12.109 with commit fd20cc68bbdb5620696ac108e8f2efd180fe2c1a
	Issue introduced in 2.6.35 with commit ddf8abd2599491cbad959c700b90ba72a5dce8d0 and fixed in 6.18.50 with commit 34e88f53614640b59039a46717fb6142e9871022
	Issue introduced in 2.6.35 with commit ddf8abd2599491cbad959c700b90ba72a5dce8d0 and fixed in 7.2.4 with commit eac233e63f9db82ba874e3cccafaafab162999e0
	Issue introduced in 2.6.35 with commit ddf8abd2599491cbad959c700b90ba72a5dce8d0 and fixed in 7.3-rc1 with commit 569dd7e5dcffe1e1c6b26ca2cd3be57eb433e082

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-2026-89732
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/gadget/function/f_fs.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/fd20cc68bbdb5620696ac108e8f2efd180fe2c1a
	https://git.kernel.org/stable/c/34e88f53614640b59039a46717fb6142e9871022
	https://git.kernel.org/stable/c/eac233e63f9db82ba874e3cccafaafab162999e0
	https://git.kernel.org/stable/c/569dd7e5dcffe1e1c6b26ca2cd3be57eb433e082