[PATCH] pci: check bus pointer before dereference

[P J P <ppandit@redhat.com>]

From: Prasad J Pandit <pjp@fedoraproject.org>

While mapping IRQ level in pci_change_irq_level() routine,
it does not check if pci_get_bus() returned a valid pointer.
It may lead to a NULL pointer dereference issue. Add check to
avoid it.

 -> https://ruhr-uni-bochum.sciebo.de/s/NNWP2GfwzYKeKwE?path=%2Flsi_nullptr1
    ==1183858==Hint: address points to the zero page.
    #0 pci_change_irq_level hw/pci/pci.c:259
    #1 pci_irq_handler hw/pci/pci.c:1445
    #2 pci_set_irq hw/pci/pci.c:1463
    #3 lsi_set_irq hw/scsi/lsi53c895a.c:488
    #4 lsi_update_irq hw/scsi/lsi53c895a.c:523
    #5 lsi_script_scsi_interrupt hw/scsi/lsi53c895a.c:554
    #6 lsi_execute_script hw/scsi/lsi53c895a.c:1149
    #7 lsi_reg_writeb hw/scsi/lsi53c895a.c:1984
    #8 lsi_io_write hw/scsi/lsi53c895a.c:2146
    ...

Reported-by: Ruhr-University <bugs-syssec@rub.de>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 hw/pci/pci.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/pci/pci.c b/hw/pci/pci.c
index de0fae10ab..df5a2c3294 100644
--- a/hw/pci/pci.c
+++ b/hw/pci/pci.c
@@ -253,6 +253,9 @@ static void pci_change_irq_level(PCIDevice *pci_dev, int irq_num, int change)
     PCIBus *bus;
     for (;;) {
         bus = pci_get_bus(pci_dev);
+        if (!bus) {
+            return;
+        }
         irq_num = bus->map_irq(pci_dev, irq_num);
         if (bus->set_irq)
             break;
-- 
2.26.2