[PATCH 2/2] PCI: of_property: Fix NULL pointer defererence in of_pci_prop_intr_map()

[Nam Cao]

The subordinate pointer can be null if we are out of bus number. The
function of_pci_prop_intr_map() deferences this pointer without checking
and may crashes the kernel.

This crash can be reproduced by starting a QEMU instance:
    qemu-system-riscv64 -machine virt \
    -kernel ../build-pci-riscv/arch/riscv/boot/Image \
    -append "console=ttyS0 root=/dev/vda" \
    -nographic \
    -drive "file=root.img,format=raw,id=hd0" \
    -device virtio-blk-device,drive=hd0 \
    -device pcie-root-port,bus=pcie.0,slot=1,id=rp1,bus-reserve=0 \
    -device pcie-pci-bridge,id=br1,bus=rp1

Then hot-add a bridge with
    device_add pci-bridge,id=br2,bus=br1,chassis_nr=1,addr=1

Then the kernel crashes:
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000028
    [snip]
[<ffffffff804dac82>] of_pci_prop_intr_map+0x104/0x362
[<ffffffff804db262>] of_pci_add_properties+0x382/0x3ca
[<ffffffff804c8228>] of_pci_make_dev_node+0xb6/0x116
[<ffffffff804a6b72>] pci_bus_add_device+0xa8/0xaa
[<ffffffff804a6ba4>] pci_bus_add_devices+0x30/0x6a
[<ffffffff804d3b5c>] shpchp_configure_device+0xa0/0x112
[<ffffffff804d2b3a>] board_added+0xce/0x354
[<ffffffff804d2e9a>] shpchp_enable_slot+0xda/0x30c
[<ffffffff804d336c>] shpchp_pushbutton_thread+0x84/0xa0

NULL check this pointer first before proceeding.

Fixes: 407d1a51921e ("PCI: Create device tree node for bridge")
Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 drivers/pci/of_property.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/pci/of_property.c b/drivers/pci/of_property.c
index 5fb516807ba2..c405978a0b7e 100644
--- a/drivers/pci/of_property.c
+++ b/drivers/pci/of_property.c
@@ -199,6 +199,9 @@ static int of_pci_prop_intr_map(struct pci_dev *pdev, struct of_changeset *ocs,
 	int ret;
 	u8 pin;
 
+	if (!pdev->subordinate)
+		return 0;
+
 	pnode = pci_device_to_OF_node(pdev->bus->self);
 	if (!pnode)
 		pnode = pci_bus_to_OF_node(pdev->bus);
-- 
2.39.2