PCI: keystone: Fix NULL pointer dereference in case of DT error in ks_pcie_setup_rc_app_regs()

If IORESOURCE_MEM is not provided in Device Tree due to
any error, resource_list_first_type() will return NULL and
pci_parse_request_of_pci_ranges() will just emit a warning.

This will cause a NULL pointer dereference. Fix this bug by adding NULL
return check.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 0f71c60 ("PCI: dwc: Remove storing of PCI resources")
Link: https://lore.kernel.org/linux-pci/20240505061517.11527-1-amishin@t-argos.ru
Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Suggested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

Author: Aleksandr Mishin

drivers/pci/controller/dwc/pci-keystone.c

@@ -400,17 +400,22 @@ static const struct irq_domain_ops ks_pcie_intx_irq_domain_ops = {
 	.xlate = irq_domain_xlate_onetwocell,
 };
 
-static void ks_pcie_setup_rc_app_regs(struct keystone_pcie *ks_pcie)
+static int ks_pcie_setup_rc_app_regs(struct keystone_pcie *ks_pcie)
 {
 	u32 val;
 	u32 num_viewport = ks_pcie->num_viewport;
 	struct dw_pcie *pci = ks_pcie->pci;
 	struct dw_pcie_rp *pp = &pci->pp;
-	u64 start, end;
+	struct resource_entry *entry;
 	struct resource *mem;
+	u64 start, end;
 	int i;
 
-	mem = resource_list_first_type(&pp->bridge->windows, IORESOURCE_MEM)->res;
+	entry = resource_list_first_type(&pp->bridge->windows, IORESOURCE_MEM);
+	if (!entry)
+		return -ENODEV;
+
+	mem = entry->res;
 	start = mem->start;
 	end = mem->end;
 
@@ -421,7 +426,7 @@ static void ks_pcie_setup_rc_app_regs(struct keystone_pcie *ks_pcie)
 	ks_pcie_clear_dbi_mode(ks_pcie);
 
 	if (ks_pcie->is_am6)
-		return;
+		return 0;
 
 	val = ilog2(OB_WIN_SIZE);
 	ks_pcie_app_writel(ks_pcie, OB_SIZE, val);
@@ -438,6 +443,8 @@ static void ks_pcie_setup_rc_app_regs(struct keystone_pcie *ks_pcie)
 	val = ks_pcie_app_readl(ks_pcie, CMD_STATUS);
 	val |= OB_XLAT_EN_VAL;
 	ks_pcie_app_writel(ks_pcie, CMD_STATUS, val);
+
+	return 0;
 }
 
 static void __iomem *ks_pcie_other_map_bus(struct pci_bus *bus,
@@ -798,7 +805,10 @@ static int __init ks_pcie_host_init(struct dw_pcie_rp *pp)
 		return ret;
 
 	ks_pcie_stop_link(pci);
-	ks_pcie_setup_rc_app_regs(ks_pcie);
+	ret = ks_pcie_setup_rc_app_regs(ks_pcie);
+	if (ret)
+		return ret;
+
 	writew(PCI_IO_RANGE_TYPE_32 | (PCI_IO_RANGE_TYPE_32 << 8),
 			pci->dbi_base + PCI_IO_BASE);