[PATCH 4/4] serial: imx: clear imx_uart_ports[] entry on probe failure and removal

[Karl Mehltretter <kmehltretter@gmail.com>]

imx_uart_probe() stores the port in imx_uart_ports[] before calling
uart_add_one_port() because console setup during that call uses the
table. The entry remains set if uart_add_one_port() fails.
imx_uart_remove() also leaves it set after uart_remove_one_port().

sport is devm-allocated, so a failed probe or unbind frees it while
imx_uart_ports[] still points to it. A later sibling probe can register
the shared console with the old preferred index and dereference the
stale entry in imx_uart_console_setup().

Reproduced on qemu's mcimx6ul-evk by unbinding a sibling UART, unbinding
the console UART and rebinding the sibling:

  BUG: KASAN: slab-use-after-free in imx_uart_console_setup+0xd0/0x3d8
  Read of size 4 at addr c49ad9d4 by task init/1
  Call trace:
   ...
   imx_uart_console_setup from try_enable_preferred_console+0x158/0x1f8
   try_enable_preferred_console from register_console+0x1cc/0x80c
   register_console from serial_core_register_port+0xe58/0xeb0
   ...
  Freed by task 1:
   ...
   devres_release_all+0x100/0x18c
   device_unbind_cleanup+0x38/0xdc
   device_release_driver_internal+0x230/0x288
   unbind_store+0x64/0xa8
   ...

The entry must remain visible while uart_add_one_port() and
uart_remove_one_port() run. Clear it when adding the port fails and
after removing the port. Save the line index before removal because the
uart_port is no longer valid afterward.

The probe-failure cleanup relies on the preceding serial-core change
"serial: core: do fallible allocations before the console can be
registered", which ensures that uart_add_one_port() cannot fail after
registering the console. For complete coverage, both changes should be
backported together.

Fixes: dbff4e9ea2e8 ("IMX UART: remove statically initialized tables")
Fixes: 9f322ad064f9 ("imx: serial: handle initialisation failure correctly")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 drivers/tty/serial/imx.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 251a50c8aa38..617c35772056 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -2637,6 +2637,8 @@ static int imx_uart_probe(struct platform_device *pdev)
 	platform_set_drvdata(pdev, sport);
 
 	ret = uart_add_one_port(&imx_uart_uart_driver, &sport->port);
+	if (ret)
+		imx_uart_ports[sport->port.line] = NULL;
 
 err_clk:
 	clk_disable_unprepare(sport->clk_ipg);
@@ -2647,8 +2649,10 @@ static int imx_uart_probe(struct platform_device *pdev)
 static void imx_uart_remove(struct platform_device *pdev)
 {
 	struct imx_port *sport = platform_get_drvdata(pdev);
+	unsigned int line = sport->port.line;
 
 	uart_remove_one_port(&imx_uart_uart_driver, &sport->port);
+	imx_uart_ports[line] = NULL;
 }
 
 static void imx_uart_restore_context(struct imx_port *sport)
-- 
2.53.0