[PATCH v2] serial: core: shut down initialized port on removal

[Igor Putko <igorpetindev@gmail.com>]

When a serial port configured as a console is opened and subsequently
closed, tty_port_shutdown() skips invoking port->ops->shutdown() because
port->console is true. As a result, tty_port_initialized() remains true
and the port's interrupt handler stays registered in the irq subsystem.

If the underlying device is later unbound or removed (e.g. via sysfs
driver unbind or device hot-unplug), serial_core_remove_one_port()
unregisters the console, frees uport->name via kfree(), and tears down
the port without shutting it down or freeing its IRQ. Consequently, the
irqaction descriptor remains registered in genirq with action->name
pointing to freed memory.

This triggers a use-after-free read whenever genirq accesses action->name,
such as when reading /proc/interrupts:

  BUG: KASAN: slab-use-after-free in string+0x476/0x570
  Call Trace:
   string+0x476/0x570
   vsnprintf+0x422/0x1300
   seq_printf+0x...
   show_interrupts+...

Fix this by acquiring port->mutex in serial_core_remove_one_port() after
console unregistration, clearing port->console under the mutex to prevent
data races with concurrent tty_port_shutdown(), and invoking
uart_shutdown(NULL, state) if the port remains initialized. This ensures
that port->ops->shutdown() is called and the IRQ is freed before
kfree(uport->name).

Reported-by: syzbot+843bf2f48f4d12e6682e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=843bf2f48f4d12e6682e
Fixes: 761ed4a94582 ("tty: serial_core: convert uart_close to use tty_port_close")
Fixes: f7048b15900f ("tty: serial_core: Add name field to uart_port struct")
Signed-off-by: Igor Putko <igorpetindev@gmail.com>
---
v1 -> v2:
- Clear port->console under port->mutex using scoped_guard() to fix
  the data race pointed out by sashiko-bot.
- Verified without synthetic devices by reproducing via sysfs driver
  unbind (00:04 serial pnp device) and inspecting /proc/interrupts.
- Do not hold port->mutex across release_port() to preserve existing
  locking semantics.

Reproducer steps (tested on QEMU with console=ttyS0):
1. Open and close the console port:
   exec 3</dev/ttyS0 && exec 3<&-
2. Unbind the serial device:
   echo "00:04" > /sys/bus/pnp/drivers/serial/unbind
3. Read interrupts:
   cat /proc/interrupts
   -> triggers KASAN use-after-free in show_interrupts() without this fix.

 drivers/tty/serial/serial_core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 95774b0f1..f6253cd30 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3213,6 +3213,13 @@ static void serial_core_remove_one_port(struct uart_driver *drv,
 	if (uart_console(uport))
 		unregister_console(uport->cons);

+	scoped_guard(mutex, &port->mutex) {
+		port->console = false;
+
+		if (tty_port_initialized(port))
+			uart_shutdown(NULL, state);
+	}
+
 	/*
 	 * Free the port IO and memory resources, if any.
 	 */
-- 
2.53.0