This reverts commit ffd603f214237e250271162a5b325c6199a65382.
Commit ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in gs_start_io") adds null pointer checks at the beginning of the gs_start_io() function to prevent a null pointer dereference. However, these checks are redundant because the function's comment already requires callers to hold the port_lock and ensure port.tty and port_usb are not null. All existing callers already follow these rules.
The true cause of the null pointer dereference is a race condition. When gs_start_io() calls either gs_start_rx() or gs_start_tx(), the port_lock is temporarily released for usb_ep_queue(). This allows port.tty and port_usb to be cleared.
Cc: stable@vger.kernel.org Fixes: ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in gs_start_io") Signed-off-by: Kuen-Han Tsai khtsai@google.com --- drivers/usb/gadget/function/u_serial.c | 6 +----- 1 file changed, 1 insertion(+), 5 deletions(-)
diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c index ab544f6824be..c043bdc30d8a 100644 --- a/drivers/usb/gadget/function/u_serial.c +++ b/drivers/usb/gadget/function/u_serial.c @@ -544,20 +544,16 @@ static int gs_alloc_requests(struct usb_ep *ep, struct list_head *head, static int gs_start_io(struct gs_port *port) { struct list_head *head = &port->read_pool; - struct usb_ep *ep; + struct usb_ep *ep = port->port_usb->out; int status; unsigned started;
- if (!port->port_usb || !port->port.tty) - return -EIO; - /* Allocate RX and TX I/O buffers. We can't easily do this much * earlier (with GFP_KERNEL) because the requests are coupled to * endpoints, as are the packet sizes we'll be using. Different * configurations may use different endpoints with a given port; * and high speed vs full speed changes packet sizes too. */ - ep = port->port_usb->out; status = gs_alloc_requests(ep, head, gs_read_complete, &port->read_allocated); if (status)
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.
Cc: stable@vger.kernel.org Fixes: 35f95fd7f234 ("TTY: usb/u_serial, use tty from tty_port") Signed-off-by: Kuen-Han Tsai khtsai@google.com --- Explanation: 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() // dereference
Stack 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
On Mon, Jun 16, 2025 at 09:21:47PM +0800, Kuen-Han Tsai wrote:
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.
Cc: stable@vger.kernel.org Fixes: 35f95fd7f234 ("TTY: usb/u_serial, use tty from tty_port") Signed-off-by: Kuen-Han Tsai khtsai@google.com
Explanation: 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() // dereference
Why isn't this up in the changelog?
Stack 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
What is [khtsai] from?
thanks,
greg k-h
On Mon, Jun 16, 2025 at 09:21:46PM +0800, Kuen-Han Tsai wrote:
This reverts commit ffd603f214237e250271162a5b325c6199a65382.
Commit ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in gs_start_io") adds null pointer checks at the beginning of the gs_start_io() function to prevent a null pointer dereference. However, these checks are redundant because the function's comment already requires callers to hold the port_lock and ensure port.tty and port_usb are not null. All existing callers already follow these rules.
The true cause of the null pointer dereference is a race condition. When gs_start_io() calls either gs_start_rx() or gs_start_tx(), the port_lock is temporarily released for usb_ep_queue(). This allows port.tty and port_usb to be cleared.
Cc: stable@vger.kernel.org Fixes: ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in gs_start_io")
As this is removing unneeded checks, why is it cc: stable? What bug is being resolved here?
confused,
greg k-h
linux-stable-mirror@lists.linaro.org
-
Greg KH -
Kuen-Han Tsai