From: Mingyu Wang <25181214217@stu.xidian.edu.cn>

A NULL pointer dereference was observed in the AMD64 AGP driver when
running in a virtualized environment (e.g., QEMU/KVM) without a physical
AMD Northbridge. The crash occurs in amd64_fetch_size() when attempting
to dereference the pointer returned by node_to_amd_nb(0).

The root cause of this crash is broken error propagation in
agp_amd64_probe(). When no AMD Northbridges are found, cache_nbs()
correctly returns -ENODEV. However, the probe function erroneously
checked the return value against exactly -1, rather than < 0.

As a result, the hardware absence error was masked, allowing the driver
to improperly proceed with initialization. It eventually called
agp_add_bridge(), which invokes amd64_fetch_size(). Since the hardware
does not exist, node_to_amd_nb(0) returns NULL, leading to a General
Protection Fault (GPF) when accessing its ->misc member.

Fix the issue by correcting the error check in agp_amd64_probe() to
abort properly when cache_nbs() returns any negative error code. This
prevents the driver from erroneously proceeding without hardware, thereby
resolving the subsequent NULL pointer dereference at its source.

Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>
---
+ Changes in v2:
+ - Dropped redundant NULL pointer checks in various initialization functions.
+ - Fixed the actual root cause: broken error propagation in agp_amd64_probe() 
+   where it erroneously checked cache_nbs() against exactly -1 instead of < 0.
+   (Thanks to Sashiko AI for the review feedback).
+
 drivers/char/agp/amd64-agp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/agp/amd64-agp.c b/drivers/char/agp/amd64-agp.c
index 2505df1f4e69..6741270e0a98 100644
--- a/drivers/char/agp/amd64-agp.c
+++ b/drivers/char/agp/amd64-agp.c
@@ -546,7 +546,7 @@ static int agp_amd64_probe(struct pci_dev *pdev,
 	/* Fill in the mode register */
 	pci_read_config_dword(pdev, bridge->capndx+PCI_AGP_STATUS, &bridge->mode);
 
-	if (cache_nbs(pdev, cap_ptr) == -1) {
+	if (cache_nbs(pdev, cap_ptr) < 0) {
 		agp_put_bridge(bridge);
 		return -ENODEV;
 	}
-- 
2.34.1