hw/pci-host/gpex: Don't fault for unmapped parts of MMIO and PIO windows

Currently the gpex PCI controller implements no special behaviour for
guest accesses to areas of the PIO and MMIO where it has not mapped
any PCI devices, which means that for Arm you end up with a CPU
exception due to a data abort.

Most host OSes expect "like an x86 PC" behaviour, where bad accesses
like this return -1 for reads and ignore writes.  In the interests of
not being surprising, make host CPU accesses to these windows behave
as -1/discard where there's no mapped PCI device.

The old behaviour generally didn't cause any problems, because
almost always the guest OS will map the PCI devices and then only
access where it has mapped them. One corner case where you will see
this kind of access is if Linux attempts to probe legacy ISA
devices via a PIO window access. So far the only case where we've
seen this has been via the syzkaller fuzzer.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 20210325163315.27724-1-peter.maydell@linaro.org
Fixes: https://bugs.launchpad.net/qemu/+bug/1918917
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

2 files changed, 56 insertions, 4 deletions

diff --git a/hw/core/machine.c b/hw/core/machine.c
index cebcdcc..0f5ce43 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -36,7 +36,9 @@
 #include "hw/virtio/virtio.h"
 #include "hw/virtio/virtio-pci.h"
 
-GlobalProperty hw_compat_6_0[] = {};
+GlobalProperty hw_compat_6_0[] = {
+    { "gpex-pcihost", "allow-unmapped-accesses", "false" },
+};
 const size_t hw_compat_6_0_len = G_N_ELEMENTS(hw_compat_6_0);
 
 GlobalProperty hw_compat_5_2[] = {
diff --git a/hw/pci-host/gpex.c b/hw/pci-host/gpex.c
index 2bdbe7b..a6752fa 100644
--- a/hw/pci-host/gpex.c
+++ b/hw/pci-host/gpex.c
@@ -83,12 +83,51 @@ static void gpex_host_realize(DeviceState *dev, Error **errp)
     int i;
 
     pcie_host_mmcfg_init(pex, PCIE_MMCFG_SIZE_MAX);
+    sysbus_init_mmio(sbd, &pex->mmio);
+
+    /*
+     * Note that the MemoryRegions io_mmio and io_ioport that we pass
+     * to pci_register_root_bus() are not the same as the
+     * MemoryRegions io_mmio_window and io_ioport_window that we
+     * expose as SysBus MRs. The difference is in the behaviour of
+     * accesses to addresses where no PCI device has been mapped.
+     *
+     * io_mmio and io_ioport are the underlying PCI view of the PCI
+     * address space, and when a PCI device does a bus master access
+     * to a bad address this is reported back to it as a transaction
+     * failure.
+     *
+     * io_mmio_window and io_ioport_window implement "unmapped
+     * addresses read as -1 and ignore writes"; this is traditional
+     * x86 PC behaviour, which is not mandated by the PCI spec proper
+     * but expected by much PCI-using guest software, including Linux.
+     *
+     * In the interests of not being unnecessarily surprising, we
+     * implement it in the gpex PCI host controller, by providing the
+     * _window MRs, which are containers with io ops that implement
+     * the 'background' behaviour and which hold the real PCI MRs as
+     * subregions.
+     */
     memory_region_init(&s->io_mmio, OBJECT(s), "gpex_mmio", UINT64_MAX);
     memory_region_init(&s->io_ioport, OBJECT(s), "gpex_ioport", 64 * 1024);
 
-    sysbus_init_mmio(sbd, &pex->mmio);
-    sysbus_init_mmio(sbd, &s->io_mmio);
-    sysbus_init_mmio(sbd, &s->io_ioport);
+    if (s->allow_unmapped_accesses) {
+        memory_region_init_io(&s->io_mmio_window, OBJECT(s),
+                              &unassigned_io_ops, OBJECT(s),
+                              "gpex_mmio_window", UINT64_MAX);
+        memory_region_init_io(&s->io_ioport_window, OBJECT(s),
+                              &unassigned_io_ops, OBJECT(s),
+                              "gpex_ioport_window", 64 * 1024);
+
+        memory_region_add_subregion(&s->io_mmio_window, 0, &s->io_mmio);
+        memory_region_add_subregion(&s->io_ioport_window, 0, &s->io_ioport);
+        sysbus_init_mmio(sbd, &s->io_mmio_window);
+        sysbus_init_mmio(sbd, &s->io_ioport_window);
+    } else {
+        sysbus_init_mmio(sbd, &s->io_mmio);
+        sysbus_init_mmio(sbd, &s->io_ioport);
+    }
+
     for (i = 0; i < GPEX_NUM_IRQS; i++) {
         sysbus_init_irq(sbd, &s->irq[i]);
         s->irq_num[i] = -1;
@@ -108,6 +147,16 @@ static const char *gpex_host_root_bus_path(PCIHostState *host_bridge,
     return "0000:00";
 }
 
+static Property gpex_host_properties[] = {
+    /*
+     * Permit CPU accesses to unmapped areas of the PIO and MMIO windows
+     * (discarding writes and returning -1 for reads) rather than aborting.
+     */
+    DEFINE_PROP_BOOL("allow-unmapped-accesses", GPEXHost,
+                     allow_unmapped_accesses, true),
+    DEFINE_PROP_END_OF_LIST(),
+};
+
 static void gpex_host_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
@@ -117,6 +166,7 @@ static void gpex_host_class_init(ObjectClass *klass, void *data)
     dc->realize = gpex_host_realize;
     set_bit(DEVICE_CATEGORY_BRIDGE, dc->categories);
     dc->fw_name = "pci";
+    device_class_set_props(dc, gpex_host_properties);
 }
 
 static void gpex_host_initfn(Object *obj)