UPSTREAM: usb: gadget: f_fs: Prevent race during ffs_ep0_queue_wait

While performing fast composition switch, there is a possibility that the
process of ffs_ep0_write/ffs_ep0_read get into a race condition
due to ep0req being freed up from functionfs_unbind.

Consider the scenario that the ffs_ep0_write calls the ffs_ep0_queue_wait
by taking a lock &ffs->ev.waitq.lock. However, the functionfs_unbind isn't
bounded so it can go ahead and mark the ep0req to NULL, and since there
is no NULL check in ffs_ep0_queue_wait we will end up in use-after-free.

Fix this by making a serialized execution between the two functions using
a mutex_lock(ffs->mutex).

Fixes: ddf8abd2 ("USB: f_fs: the FunctionFS driver")
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
Tested-by: Krishna Kurapati <quic_kriskura@quicinc.com>
Link: https://lore.kernel.org/r/20221215052906.8993-2-quic_ugoswami@quicinc.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Bug: 265837309
Bug: 269722315
(cherry picked from commit 6a19da11 https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git/

 usb-linus)
Change-Id: I1d71b96709a557caee72d1c84e3ee43969a89c13
Signed-off-by: Prashanth K <quic_prashk@quicinc.com>
Signed-off-by: Yinchu Chen <chenyc5@motorola.com>