[PATCH 1/2] softmmu/physmem: move last_ram_page() call under qemu_mutex_lock_ramlist()

[Andrey Ryabinin]

The 'ram_list.blocks' modifications protected by 'ram_list.mutex'.
last_ram_page() uses state of 'ram_list.blocks' to identify ram's size.
ram_block_add() calls last_ram_page() before the mutex lock
making the following race possible:

     CPU#0                                       CPU#1
                                      ram_block_add()
                                         old_ram_size = last_ram_page()
                                         qemu_mutex_lock_ramlist()
                                         ...
                                         dirty_memory_extend(old_ram_size, 
new_ram_size);
ram_block_add()
   old_ram_size = last_ram_page()

                                              //insert block to ram_list
                                              QLIST_INSERT_*_RCU()
                                              qemu_mutex_unlock_ramlist()
   qemu_mutex_lock_ramlist()
   ....
   dirty_memory_extend(old_ram_size, new_ram_size);

Such race may result in leaking some dirty memory bitmaps.

Because of stale 'old_ram_size' value, the dirty_memory_extend() on CPU#0
will allocate and reinitialize some of the already allocated on CPU#1
dirty memory bitmap blocks.

Fix this by moving last_ram_page() call under the qemu_mutex_lock_ramlist()

Cc: qemu-sta...@nongnu.org
Signed-off-by: Andrey Ryabinin <a...@yandex-team.com>
---
 softmmu/physmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index 4e1b27a20e..32f76362bf 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -1969,9 +1969,9 @@ static void ram_block_add(RAMBlock *new_block, Error 
**errp)
     ram_addr_t old_ram_size, new_ram_size;
     Error *err = NULL;
 
+    qemu_mutex_lock_ramlist();
     old_ram_size = last_ram_page();
 
-    qemu_mutex_lock_ramlist();
     new_block->offset = find_ram_offset(new_block->max_length);
 
     if (!new_block->host) {
-- 
2.34.1