display/bochs: fix pcie support

Set QEMU_PCI_CAP_EXPRESS unconditionally in init(), then clear it in
realize() in case the device is not connected to a PCIe bus.

This makes sure the pci config space allocation is big enough, so
accessing the PCIe extended config space doesn't overflow the pci
config space buffer.

PCI(e) config space is guest writable.  Writes are limited by
write mask (which probably is also filled with random stuff),
so the guest can only flip enabled bits.  But I suspect it
still might be exploitable, so rather serious because it might
be a host escape for the guest.  On the other hand the device
is probably not yet in widespread use.

(For a QEMU version without this commit, a mitigation for the
bug is available: use "-device bochs-display" as a conventional pci
device only.)

Cc: qemu-stable@nongnu.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20190812065221.20907-2-kraxel@redhat.com
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/display/bochs-display.c

@@ -297,9 +297,10 @@ static void bochs_display_realize(PCIDevice *dev, Error **errp)
     }
 
     if (pci_bus_is_express(pci_get_bus(dev))) {
-        dev->cap_present |= QEMU_PCI_CAP_EXPRESS;
         ret = pcie_endpoint_cap_init(dev, 0x80);
         assert(ret > 0);
+    } else {
+        dev->cap_present &= ~QEMU_PCI_CAP_EXPRESS;
     }
 
     memory_region_set_log(&s->vram, true, DIRTY_MEMORY_VGA);
@@ -322,11 +323,15 @@ static void bochs_display_set_big_endian_fb(Object *obj, bool value,
 
 static void bochs_display_init(Object *obj)
 {
+    PCIDevice *dev = PCI_DEVICE(obj);
+
     /* Expose framebuffer byteorder via QOM */
     object_property_add_bool(obj, "big-endian-framebuffer",
                              bochs_display_get_big_endian_fb,
                              bochs_display_set_big_endian_fb,
                              NULL);
+
+    dev->cap_present |= QEMU_PCI_CAP_EXPRESS;
 }
 
 static void bochs_display_exit(PCIDevice *dev)