[PATCH v2 2/4] serial: core: clear freed pointers on uart_register_driver() failure

[Karl Mehltretter <kmehltretter@gmail.com>]

uart_register_driver() leaves freed pointers behind on failure. If
tty_alloc_driver() fails, it frees drv->state without clearing it. If
tty_register_driver() fails, it also drops the tty driver reference
without clearing drv->tty_driver.

Several drivers register the uart_driver lazily and use drv->state as
an "already registered" sentinel. After a failed registration, the
next probe sees the stale pointer, skips re-registration and calls
uart_add_one_port() with freed state.

The resulting unwind can call uart_unregister_driver() with a NULL or
dangling drv->tty_driver and oops in tty_unregister_driver():

  Unhandled fault: page domain fault (0x01b) at 0x00000018
  PC is at tty_unregister_driver+0x10/0x68
  LR is at uart_unregister_driver+0x1c/0x60

Reproduced with failslab fail-nth injection on qemu's raspi1ap board:
fail the tty_alloc_driver() allocation during a sysfs bind of the
PL011 port, then bind again in the same boot.

Clear drv->state after freeing the state array and clear
drv->tty_driver after dropping the tty driver reference, as
uart_unregister_driver() already does.

The tty_register_driver() failure case predates Git history. The
tty_alloc_driver() failure case was introduced by commit 9e845abfc8a8
("serial: fix NULL pointer dereference"), which made that error path
return cleanly instead of crashing in put_tty_driver(NULL).

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: 9e845abfc8a8 ("serial: fix NULL pointer dereference")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 drivers/tty/serial/serial_core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 887b1dd80ad2..ba9145c5a38a 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2777,8 +2777,10 @@ int uart_register_driver(struct uart_driver *drv)
 	for (i = 0; i < drv->nr; i++)
 		tty_port_destroy(&drv->state[i].port);
 	tty_driver_kref_put(normal);
+	drv->tty_driver = NULL;
 out_kfree:
 	kfree(drv->state);
+	drv->state = NULL;
 out:
 	return retval;
 }
-- 
2.53.0