PCI: host-generic: Fix NULL pointer dereference on 32-bit CAM systems

On 32-bit systems the config space is too large to ioremap in one go, so
pci_ecam_create() maps each bus segment separately and relies on the
->add_bus callback (pci_ecam_add_bus) to populate the per-bus mapping in
cfg->winp[]. pci_ecam_map_bus() then uses that mapping as the base for
every config access.

The generic ECAM ops (pci_generic_ecam_ops) already provide the ->add_bus
and ->remove_bus callbacks, but the CAM (legacy) ops in pci-host-generic.c
do not. As a result, on a 32-bit host using "pci-host-cam-generic" the
per-bus mapping is never set up and the first config read dereferences a
NULL base, crashing during bus enumeration:

 Unable to handle kernel NULL pointer dereference at virtual address 00000800
 Oops [#1]
 CPU: 0 PID: 1 Comm: swapper Not tainted 6.9.7+ #43
 Hardware name: Digilent Nexys-Video-A7 RV32 (DT)
 epc : pci_generic_config_read+0x40/0xb0
  ra : pci_generic_config_read+0x2c/0xb0
 [<c038db9c>] pci_generic_config_read+0x40/0xb0
 [<c038da04>] pci_bus_read_config_dword+0x50/0xb0
 [<c0391e94>] pci_bus_generic_read_dev_vendor_id+0x3c/0x1ec
 [<c039245c>] pci_scan_single_device+0xa4/0x11c
 [<c0392570>] pci_scan_slot+0x9c/0x23c
 [<c039388c>] pci_scan_child_bus_extend+0x58/0x2f4
 [<c0393db0>] pci_scan_root_bus_bridge+0x64/0xe8
 [<c0393e54>] pci_host_probe+0x20/0xc8
 [<c03bc6f4>] pci_host_common_probe+0x144/0x1e4

Fix this by giving the CAM ops the same ->add_bus/->remove_bus callbacks.
Since pci_ecam_add_bus() and pci_ecam_remove_bus() are static to ecam.c,
move the CAM ops definition there as pci_generic_cam_ops (mirroring
pci_generic_ecam_ops) and export it for pci-host-generic.c to reference.

Fixes: 8fe55ef233 ("PCI: Dynamically map ECAM regions")
Signed-off-by: Steffen Persvold <spersvold@gmail.com>
[mani: removed timestamp from log]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260709122446.3151899-1-spersvold@gmail.com

drivers/pci/controller/pci-host-generic.c

@@ -16,15 +16,6 @@
 
 #include "pci-host-common.h"
 
-static const struct pci_ecam_ops gen_pci_cfg_cam_bus_ops = {
-	.bus_shift	= 16,
-	.pci_ops	= {
-		.map_bus	= pci_ecam_map_bus,
-		.read		= pci_generic_config_read,
-		.write		= pci_generic_config_write,
-	}
-};
-
 static bool pci_dw_valid_device(struct pci_bus *bus, unsigned int devfn)
 {
 	struct pci_config_window *cfg = bus->sysdata;
@@ -60,7 +51,7 @@ static const struct pci_ecam_ops pci_dw_ecam_bus_ops = {
 
 static const struct of_device_id gen_pci_of_match[] = {
 	{ .compatible = "pci-host-cam-generic",
-	  .data = &gen_pci_cfg_cam_bus_ops },
+	  .data = &pci_generic_cam_ops },
 
 	{ .compatible = "pci-host-ecam-generic",
 	  .data = &pci_generic_ecam_ops },

drivers/pci/ecam.c

@@ -208,6 +208,19 @@ const struct pci_ecam_ops pci_generic_ecam_ops = {
 };
 EXPORT_SYMBOL_GPL(pci_generic_ecam_ops);
 
+/* CAM ops */
+const struct pci_ecam_ops pci_generic_cam_ops = {
+	.bus_shift	= 16,
+	.pci_ops	= {
+		.add_bus	= pci_ecam_add_bus,
+		.remove_bus	= pci_ecam_remove_bus,
+		.map_bus	= pci_ecam_map_bus,
+		.read		= pci_generic_config_read,
+		.write		= pci_generic_config_write,
+	}
+};
+EXPORT_SYMBOL_GPL(pci_generic_cam_ops);
+
 #if defined(CONFIG_ACPI) && defined(CONFIG_PCI_QUIRKS)
 /* ECAM ops for 32-bit access only (non-compliant) */
 const struct pci_ecam_ops pci_32b_ops = {

include/linux/pci-ecam.h

@@ -81,6 +81,9 @@ void __iomem *pci_ecam_map_bus(struct pci_bus *bus, unsigned int devfn,
 /* default ECAM ops */
 extern const struct pci_ecam_ops pci_generic_ecam_ops;
 
+/* default CAM ops */
+extern const struct pci_ecam_ops pci_generic_cam_ops;
+
 #if defined(CONFIG_ACPI) && defined(CONFIG_PCI_QUIRKS)
 extern const struct pci_ecam_ops pci_32b_ops;	/* 32-bit accesses only */
 extern const struct pci_ecam_ops pci_32b_read_ops; /* 32-bit read only */