linux-kernel - Re: [PATCH] platform/x86: dell-uart-backlight: fix NULL pointer dereference in probe

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Message-ID: <0cb76614-fdba-47aa-b4c8-f001a303406f@redhat.com>
Date: Sat, 11 Jan 2025 10:29:36 +0100
From: Hans de Goede <hdegoede@...hat.com>
To: Chenyuan Yang <chenyuan0y@...il.com>, ilpo.jarvinen@...ux.intel.com,
 rafael.j.wysocki@...el.com, acelan.kao@...onical.com,
 u.kleine-koenig@...libre.com
Cc: platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
 zijie98@...il.com
Subject: Re: [PATCH] platform/x86: dell-uart-backlight: fix NULL pointer
 dereference in probe

Hi,

On 11-Jan-25 6:47 AM, Chenyuan Yang wrote:
> The dell_uart_bl_serdev_probe() function calls devm_serdev_device_open()
> before setting the client ops via serdev_device_set_client_ops(). This
> ordering can trigger a NULL pointer dereference in the serdev controller's
> receive_buf handler, as it assumes serdev->ops is valid when
> SERPORT_ACTIVE is set.
> 
> This is similar to the issue fixed in commit 5e700b384ec1
> ("platform/chrome: cros_ec_uart: properly fix race condition") where
> devm_serdev_device_open() was called before fully initializing the
> device.
> 
> Fix the race by ensuring client ops are set before enabling the port via
> devm_serdev_device_open().
> 
> Fixes: 484bae9e4d6a ("platform/x86: Add new Dell UART backlight driver")
> Signed-off-by: Chenyuan Yang <chenyuan0y@...il.com>

Thank you for your patch.

> ---
>  drivers/platform/x86/dell/dell-uart-backlight.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
> index 6e5dc7e3674f..41162ff1686b 100644
> --- a/drivers/platform/x86/dell/dell-uart-backlight.c
> +++ b/drivers/platform/x86/dell/dell-uart-backlight.c
> @@ -283,16 +283,16 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
>  	init_waitqueue_head(&dell_bl->wait_queue);
>  	dell_bl->dev = dev;
>  
> -	ret = devm_serdev_device_open(dev, serdev);
> -	if (ret)
> -		return dev_err_probe(dev, ret, "opening UART device\n");
> -
>  	/* 9600 bps, no flow control, these are the default but set them to be sure */
>  	serdev_device_set_baudrate(serdev, 9600);
>  	serdev_device_set_flow_control(serdev, false);
>  	serdev_device_set_drvdata(serdev, dell_bl);
>  	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
>  
> +	ret = devm_serdev_device_open(dev, serdev);
> +	if (ret)
> +		return dev_err_probe(dev, ret, "opening UART device\n");
> +

serdev_device_open() resets the termios settings such as flow-control,
so you need to move the serdev_device_set_baudrate() and
serdev_device_set_flow_control() calls to after the new location of
the devm_serdev_device_open() call, just like commit 5e700b384ec1
("platform/chrome: cros_ec_uart: properly fix race condition") does.

Regards,

Hans



>  	get_version[0] = DELL_SOF(GET_CMD_LEN);
>  	get_version[1] = CMD_GET_VERSION;
>  	get_version[2] = dell_uart_checksum(get_version, 2);