FAILED: patch "[PATCH] cxl/acpi: Check ACPI companion before use" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 371c2ad1624283b6c7a62a28199f5161d388a738
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107060040-failed-5.15-371c2ad16242@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 371c2ad1624283b6c7a62a28199f5161d388a738 Mon Sep 17 00:00:00 2001
From: Jiale Yao <yaojiale02@163.com>
Date: Sat, 26 Sep 2026 15:16:05 +0800
Subject: [PATCH] cxl/acpi: Check ACPI companion before use

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>
Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
Link: https://patch.msgid.link/20260926071605.3013893-1-yaojiale02@163.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
---
 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 3b818adbd38b5..fb09a5ff48c16 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,
-- 
2.53.0