Re: [PATCH v2] cxl/acpi: Check ACPI companion before use

[Jonathan Cameron]

On Sat, 26 Sep 2026 15:16:05 +0800
Jiale Yao <yaojiale02@xxxxxxx> wrote:

> Platform drivers can be forced to match devices outside their ID tables
> through driver_override.  cxl_acpi_probe() assumes that every bound device
> has an ACPI companion and dereferences adev->dev.bus without checking the
> result of ACPI_COMPANION().  Force-binding cxl_acpi to a platform device
> without a companion therefore causes a NULL pointer dereference.
> 
Given I had this drawn to my attention earlier (thanks to Uwe)
let me draw attention to an attempt to add a flag to remove the need
for this sort of defense by blocking driver_override.

https://lore.kernel.org/lkml/20260922-driver-override-opt-out-v1-0-58c35ded3b83@xxxxxxxxxx/

So maybe we can replace what we do here once that lands.
In the meantime it's a valid bug so 

Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>

> This was reproduced by setting the driver override for the pcspkr platform
> device to cxl_acpi and binding it through sysfs:
> 
>   BUG: kernel NULL pointer dereference, address: 0000000000000280
>   #PF: supervisor read access in kernel mode
>   RIP: cxl_acpi_probe+0xf4/0x220
>   Call Trace:
>    platform_probe+0x4d/0x80
>    really_probe+0x106/0x370
>    device_driver_attach+0x4c/0xa0
>    bind_store+0xd0/0x100
> 
> Commit 2b3a5dabe89e ("platform/surface: acpi-notify: Check ACPI
> companion before use") fixed the same force-binding issue in another
> platform driver.  Check the companion before setting up the CXL root and
> return -ENODEV when it is absent.
> 
> Fixes: 7d4b5ca2e2cb ("cxl/acpi: Add downstream port data to cxl_port instances")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jiale Yao <yaojiale02@xxxxxxx>
> ---
> 
> Notes:
>     Changes in v2:
>     - Move the ACPI companion assignment immediately before its NULL check.
>     - Reorder local declarations in reverse Christmas tree order.
> 
>  drivers/cxl/acpi.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 3b818adbd38b..fb09a5ff48c1 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -885,13 +885,17 @@ static int pair_cxl_resource(struct device *dev, void *data)
>  
>  static int cxl_acpi_probe(struct platform_device *pdev)
>  {
> -	int rc;
> +	struct cxl_cfmws_context ctx;
> +	struct acpi_device *adev;
>  	struct resource *cxl_res;
>  	struct cxl_root *cxl_root;
>  	struct cxl_port *root_port;
>  	struct device *host = &pdev->dev;
> -	struct acpi_device *adev = ACPI_COMPANION(host);
> -	struct cxl_cfmws_context ctx;
> +	int rc;
> +
> +	adev = ACPI_COMPANION(host);
> +	if (!adev)
> +		return -ENODEV;
>  
>  	device_lock_set_class(&pdev->dev, &cxl_root_key);
>  	rc = devm_add_action_or_reset(&pdev->dev, cxl_acpi_lock_reset_class,