softmmu/ioport.c: make MemoryRegionPortioList owner of portio_list MemoryRegions

Currently when portio_list MemoryRegions are freed using portio_list_destroy() the RCU
thread segfaults generating a backtrace similar to that below:

    #0 0x5555599a34b6 in phys_section_destroy ../softmmu/physmem.c:996
    #1 0x5555599a37a3 in phys_sections_free ../softmmu/physmem.c:1011
    #2 0x5555599b24aa in address_space_dispatch_free ../softmmu/physmem.c:2430
    #3 0x55555996a283 in flatview_destroy ../softmmu/memory.c:292
    #4 0x55555a2cb9fb in call_rcu_thread ../util/rcu.c:284
    #5 0x55555a29b71d in qemu_thread_start ../util/qemu-thread-posix.c:541
    #6 0x7ffff4a0cea6 in start_thread nptl/pthread_create.c:477
    #7 0x7ffff492ca2e in __clone (/lib/x86_64-linux-gnu/libc.so.6+0xfca2e)

The problem here is that portio_list_destroy() unparents the portio_list
MemoryRegions causing them to be freed immediately, however the flatview
still has a reference to the MemoryRegion and so causes a use-after-free
segfault when the RCU thread next updates the flatview.

Solve the lifetime issue by making MemoryRegionPortioList the owner of the
portio_list MemoryRegions, and then reparenting them to the portio_list
owner. This ensures that they can be accessed as QOM children via the
portio_list owner, yet the MemoryRegionPortioList owns the refcount.

Update portio_list_destroy() to unparent the MemoryRegion from the
portio_list owner (while keeping mrpio->mr live until finalization of the
MemoryRegionPortioList), so that the portio_list MemoryRegions remain
allocated until flatview_destroy() removes the final refcount upon the
next flatview update.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230419151652.362717-4-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

softmmu/ioport.c

@@ -229,6 +229,8 @@ static void portio_list_add_1(PortioList *piolist,
                               unsigned off_low, unsigned off_high)
 {
     MemoryRegionPortioList *mrpio;
+    Object *owner;
+    char *name;
     unsigned i;
 
     /* Copy the sub-list and null-terminate it.  */
@@ -245,8 +247,25 @@ static void portio_list_add_1(PortioList *piolist,
         mrpio->ports[i].base = start + off_low;
     }
 
-    memory_region_init_io(&mrpio->mr, piolist->owner, &portio_ops, mrpio,
+    /*
+     * The MemoryRegion owner is the MemoryRegionPortioList since that manages
+     * the lifecycle via the refcount
+     */
+    memory_region_init_io(&mrpio->mr, OBJECT(mrpio), &portio_ops, mrpio,
                           piolist->name, off_high - off_low);
+
+    /* Reparent the MemoryRegion to the piolist owner */
+    object_ref(&mrpio->mr);
+    object_unparent(OBJECT(&mrpio->mr));
+    if (!piolist->owner) {
+        owner = container_get(qdev_get_machine(), "/unattached");
+    } else {
+        owner = piolist->owner;
+    }
+    name = g_strdup_printf("%s[*]", piolist->name);
+    object_property_add_child(owner, name, OBJECT(&mrpio->mr));
+    g_free(name);
+
     if (piolist->flush_coalesced_mmio) {
         memory_region_set_flush_coalesced(&mrpio->mr);
     }
@@ -308,6 +327,7 @@ static void memory_region_portio_list_finalize(Object *obj)
 {
     MemoryRegionPortioList *mrpio = MEMORY_REGION_PORTIO_LIST(obj);
 
+    object_unref(&mrpio->mr);
     g_free(mrpio->ports);
 }