A race condition occurs when gs_start_io() calls either gs_start_rx() or gs_start_tx(), as those functions briefly drop the port_lock for usb_ep_queue(). This allows gs_close() and gserial_disconnect() to clear port.tty and port_usb, respectively.

Use the null-safe TTY Port helper function to wake up TTY.

Example CPU1: CPU2: gserial_connect() // lock gs_close() // await lock gs_start_rx() // unlock usb_ep_queue() gs_close() // lock, reset port.tty and unlock gs_start_rx() // lock tty_wakeup() // NPE

Fixes: 35f95fd7f234 ("TTY: usb/u_serial, use tty from tty_port") Cc: stable@vger.kernel.org Signed-off-by: Kuen-Han Tsai khtsai@google.com --- v2: - Move the example up to the changelog

Traces: [ 51.494375][ T278] ttyGS1: shutdown [ 51.494817][ T269] android_work: sent uevent USB_STATE=DISCONNECTED [ 52.115792][ T1508] usb: [dm_bind] generic ttyGS1: super speed IN/ep1in OUT/ep1out [ 52.516288][ T1026] android_work: sent uevent USB_STATE=CONNECTED [ 52.551667][ T1533] gserial_connect: start ttyGS1 [ 52.565634][ T1533] [khtsai] enter gs_start_io, ttyGS1, port->port.tty=0000000046bd4060 [ 52.565671][ T1533] [khtsai] gs_start_rx, unlock port ttyGS1 [ 52.591552][ T1533] [khtsai] gs_start_rx, lock port ttyGS1 [ 52.619901][ T1533] [khtsai] gs_start_rx, unlock port ttyGS1 [ 52.638659][ T1325] [khtsai] gs_close, lock port ttyGS1 [ 52.656842][ T1325] gs_close: ttyGS1 (0000000046bd4060,00000000be9750a5) ... [ 52.683005][ T1325] [khtsai] gs_close, clear ttyGS1 [ 52.683007][ T1325] gs_close: ttyGS1 (0000000046bd4060,00000000be9750a5) done! [ 52.708643][ T1325] [khtsai] gs_close, unlock port ttyGS1 [ 52.747592][ T1533] [khtsai] gs_start_rx, lock port ttyGS1 [ 52.747616][ T1533] [khtsai] gs_start_io, ttyGS1, going to call tty_wakeup(), port->port.tty=0000000000000000 [ 52.747629][ T1533] Unable to handle kernel NULL pointer dereference at virtual address 00000000000001f8 --- drivers/usb/gadget/function/u_serial.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c index c043bdc30d8a..540dc5ab96fc 100644 --- a/drivers/usb/gadget/function/u_serial.c +++ b/drivers/usb/gadget/function/u_serial.c @@ -295,8 +295,8 @@ __acquires(&port->port_lock) break; }

- if (do_tty_wake && port->port.tty) - tty_wakeup(port->port.tty); + if (do_tty_wake) + tty_port_tty_wakeup(&port->port); return status; }

@@ -574,7 +574,7 @@ static int gs_start_io(struct gs_port *port) gs_start_tx(port); /* Unblock any pending writes into our circular buffer, in case * we didn't in gs_start_tx() */ - tty_wakeup(port->port.tty); + tty_port_tty_wakeup(&port->port); } else { /* Free reqs only if we are still connected */ if (port->port_usb) { -- 2.50.0.rc2.692.g299adb8693-goog