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

[Jiale Yao <yaojiale02@163.com>]

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.

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@vger.kernel.org
Signed-off-by: Jiale Yao <yaojiale02@163.com>
---

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,
-- 
2.34.1