[PATCH] PCI: hotplug: shpchp: Prevent NULL pointer dereference during probe

[Nam Cao]

pci_dev->subordinate pointer can be NULL if we run out of bus number. The
driver deferences this pointer without checking, and the kernel crashes.

This crash can be reproduced by starting a QEMU instance:
    qemu-system-x86_64 -machine pc-q35-2.10 \
    -kernel bzImage \
    -drive "file=img,format=raw" \
    -m 2048 -smp 1 -enable-kvm \
    -append "console=ttyS0 root=/dev/sda debug" \
    -nographic \
    -device pcie-root-port,bus=pcie.0,slot=1,id=rp1 \
    -device pcie-pci-bridge,id=br1,bus=rp1

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

Then the kernel crashes:
shpchp 0000:02:01.0: enabling device (0000 -> 0002)
shpchp 0000:02:01.0: enabling bus mastering
BUG: kernel NULL pointer dereference, address: 00000000000000da
    [snip]
Call Trace:
 <TASK>
 ? show_regs+0x63/0x70
 ? __die+0x23/0x70
 ? page_fault_oops+0x17a/0x480
 ? shpc_init+0x3fb/0x9d0
 ? search_module_extables+0x4e/0x80
 ? shpc_init+0x3fb/0x9d0
 ? kernelmode_fixup_or_oops+0x9b/0x120
 ? __bad_area_nosemaphore+0x16e/0x240
 ? bad_area_nosemaphore+0x11/0x20
 ? do_user_addr_fault+0x2a3/0x610
 ? exc_page_fault+0x6d/0x160
 ? asm_exc_page_fault+0x2b/0x30
 ? shpc_init+0x3fb/0x9d0
 shpc_probe+0x92/0x390

NULL check this pointer first before proceeding. If there is no
secondary bus number, there is no point in initializing this hot-plug
controller, so just bails out.

Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # all
---
This one exists since beginning of git history. So I didn't bother
with a Fixes: tag.

This patch is almost a copy-paste from pciehp
---
 drivers/pci/hotplug/shpchp_core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/pci/hotplug/shpchp_core.c b/drivers/pci/hotplug/shpchp_core.c
index 56c7795ed890..14cf9e894201 100644
--- a/drivers/pci/hotplug/shpchp_core.c
+++ b/drivers/pci/hotplug/shpchp_core.c
@@ -262,6 +262,12 @@ static int shpc_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 	if (acpi_get_hp_hw_control_from_firmware(pdev))
 		return -ENODEV;
 
+	if (!pdev->subordinate) {
+		/* Can happen if we run out of bus numbers during probe */
+		pci_err(pdev, "Hotplug bridge without secondary bus, ignoring\n");
+		return -ENODEV;
+	}
+
 	ctrl = kzalloc(sizeof(*ctrl), GFP_KERNEL);
 	if (!ctrl)
 		goto err_out_none;
-- 
2.39.2