[PATCH 3/3] softmmu/ioport.c: make MemoryRegionPortioList owner of portio_list MemoryRegions

[Mark Cave-Ayland]

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 childen via the portio_list owner, yet the 
MemoryRegionPortioList
owns the refcount.

Update portio_list_destroy() to unparent the MemoryRegion from the portio_list 
owner and
then add a finalize() method to 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>
---
 softmmu/ioport.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/softmmu/ioport.c b/softmmu/ioport.c
index 238625a36f..d89c659662 100644
--- a/softmmu/ioport.c
+++ b/softmmu/ioport.c
@@ -26,6 +26,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/rcu.h"
 #include "cpu.h"
 #include "exec/ioport.h"
 #include "exec/memory.h"
@@ -152,8 +153,7 @@ void portio_list_destroy(PortioList *piolist)
     for (i = 0; i < piolist->nr; ++i) {
         mrpio = container_of(piolist->regions[i], MemoryRegionPortioList, mr);
         object_unparent(OBJECT(&mrpio->mr));
-        g_free(mrpio->ports);
-        g_free(mrpio);
+        object_unref(mrpio);
     }
     g_free(piolist->regions);
 }
@@ -230,6 +230,8 @@ 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.  */
@@ -246,8 +248,25 @@ 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);
     }
@@ -305,10 +324,19 @@ void portio_list_del(PortioList *piolist)
     }
 }
 
+static void memory_region_portio_list_finalize(Object *obj)
+{
+    MemoryRegionPortioList *mrpio = MEMORY_REGION_PORTIO_LIST(obj);
+
+    object_unref(&mrpio->mr);
+    g_free(mrpio->ports);
+}
+
 static const TypeInfo memory_region_portio_list_info = {
     .parent             = TYPE_OBJECT,
     .name               = TYPE_MEMORY_REGION_PORTIO_LIST,
     .instance_size      = sizeof(MemoryRegionPortioList),
+    .instance_finalize  = memory_region_portio_list_finalize,
 };
 
 static void ioport_register_types(void)
-- 
2.30.2