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@xxxxxxxxxxxxxxxxxxx> 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
