USB: gadget: Fix obscure lockdep violation for udc_mutex

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

USB: gadget: Fix obscure lockdep violation for udc_mutex

A recent commit expanding the scope of the udc_lock mutex in the

gadget core managed to cause an obscure and slightly bizarre lockdep

violation. In abbreviated form:

======================================================

WARNING: possible circular locking dependency detected

5.19.0-rc7+ #12510 Not tainted

------------------------------------------------------

udevadm/312 is trying to acquire lock:

ffff80000aae1058 (udc_lock){+.+.}-{3:3}, at: usb_udc_uevent+0x54/0xe0

but task is already holding lock:

ffff000002277548 (kn->active#4){++++}-{0:0}, at: kernfs_seq_start+0x34/0xe0

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (kn->active#4){++++}-{0:0}:

lock_acquire+0x68/0x84

__kernfs_remove+0x268/0x380

kernfs_remove_by_name_ns+0x58/0xac

sysfs_remove_file_ns+0x18/0x24

device_del+0x15c/0x440

-> #2 (device_links_lock){+.+.}-{3:3}:

lock_acquire+0x68/0x84

__mutex_lock+0x9c/0x430

mutex_lock_nested+0x38/0x64

device_link_remove+0x3c/0xa0

_regulator_put.part.0+0x168/0x190

regulator_put+0x3c/0x54

devm_regulator_release+0x14/0x20

-> #1 (regulator_list_mutex){+.+.}-{3:3}:

lock_acquire+0x68/0x84

__mutex_lock+0x9c/0x430

mutex_lock_nested+0x38/0x64

regulator_lock_dependent+0x54/0x284

regulator_enable+0x34/0x80

phy_power_on+0x24/0x130

__dwc2_lowlevel_hw_enable+0x100/0x130

dwc2_lowlevel_hw_enable+0x18/0x40

dwc2_hsotg_udc_start+0x6c/0x2f0

gadget_bind_driver+0x124/0x1f4

-> #0 (udc_lock){+.+.}-{3:3}:

__lock_acquire+0x1298/0x20cc

lock_acquire.part.0+0xe0/0x230

lock_acquire+0x68/0x84

__mutex_lock+0x9c/0x430

mutex_lock_nested+0x38/0x64

usb_udc_uevent+0x54/0xe0

Evidently this was caused by the scope of udc_mutex being too large.

The mutex is only meant to protect udc->driver along with a few other

things. As far as I can tell, there's no reason for the mutex to be

held while the gadget core calls a gadget driver's ->bind or ->unbind

routine, or while a UDC is being started or stopped. (This accounts

for link #1 in the chain above, where the mutex is held while the

dwc2_hsotg_udc is started as part of driver probing.)

Gadget drivers' ->disconnect callbacks are problematic. Even though

usb_gadget_disconnect() will now acquire the udc_mutex, there's a

window in usb_gadget_bind_driver() between the times when the mutex is

released and the ->bind callback is invoked. If a disconnect occurred

during that window, we could call the driver's ->disconnect routine

before its ->bind routine. To prevent this from happening, it will be

necessary to prevent a UDC from connecting while it has no gadget

driver. This should be done already but it doesn't seem to be;

currently usb_gadget_connect() has no check for this. Such a check

will have to be added later.

Some degree of mutual exclusion is required in soft_connect_store(),

which can dereference udc->driver at arbitrary times since it is a

sysfs callback. The solution here is to acquire the gadget's device

lock rather than the udc_mutex. Since the driver core guarantees that

the device lock is always held during driver binding and unbinding,

this will make the accesses in soft_connect_store() mutually exclusive

with any changes to udc->driver.

Lastly, it turns out there is one place which should hold the

udc_mutex but currently does not: The function_show() routine needs

protection while it dereferences udc->driver. The missing lock and

unlock calls are added.