Re: [PATCH] usb: gadget: u_serial: Add null pointer checks after RX/TX submission

[Jiri Slaby <jirislaby@kernel.org>]

On 08. 03. 24, 12:47, Kuen-Han Tsai wrote:
> Hi Greg & Jiri,
> 
> On Sun, Jan 28, 2024 at 9:29 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>>
>> On Thu, Jan 18, 2024 at 10:27:54AM +0100, Jiri Slaby wrote:
>>> On 16. 01. 24, 15:16, Kuen-Han Tsai wrote:
>>>> Commit ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in
>>>> gs_start_io") adds null pointer checks to gs_start_io(), but it doesn't
>>>> fully fix the potential null pointer dereference issue. While
>>>> gserial_connect() calls gs_start_io() with port_lock held, gs_start_rx()
>>>> and gs_start_tx() release the lock during endpoint request submission.
>>>> This creates a window where gs_close() could set port->port_tty to NULL,
>>>> leading to a dereference when the lock is reacquired.
>>>>
>>>> This patch adds a null pointer check for port->port_tty after RX/TX
>>>> submission, and removes the initial null pointer check in gs_start_io()
>>>> since the caller must hold port_lock and guarantee non-null values for
>>>> port_usb and port_tty.
>>>
>>> Or you switch to tty_port refcounting and need not fiddling with this at all
>>> ;).
>>
>> I agree, Kuen-Han, why not do that instead?
> 
> The u_serial driver has already maintained the usage count of a TTY
> structure for open and close. While the driver tracks the usage count
> via open/close, it doesn't fully eliminate race conditions. Below are
> two potential scenarios:
> 
> Case 1 (Observed):
> 1. gs_open() sets usage count to 1.
> 2. gserial_connect(), gs_start_io(), and gs_start_rx() execute in
> sequence (lock held).
> 3. Lock released, usb_ep_queue() called.
> 4. In parallel, gs_close() executes, sees count of 1, clears TTY, releases lock.
> 5. Original thread resumes in gs_start_rx(), potentially leading to
> kernel panic on an invalid TTY.

If it used refcounting -- tty_port_tty_get(), how comes?

thanks,
-- 
js
suse labs