[Qemu-devel] [PATCH] exec: fix access to ram_list.dirty_memory when sync dirty bitmap

[Haozhong Zhang <haozhong.zhang@intel.com>]

In cpu_physical_memory_sync_dirty_bitmap(rb, start, ...), the 2nd
argument 'start' is relative to the start of the ramblock 'rb'. When
it's used to access the dirty memory bitmap of ram_list (i.e.
ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]->blocks[]), an offset to
the start of all RAM (i.e. rb->offset) should be added to it, which has
however been missed since c/s 6b6712efcc. For a ramblock of host memory
backend whose offset is not zero, cpu_physical_memory_sync_dirty_bitmap()
synchronizes the incorrect part of the dirty memory bitmap of ram_list
to the per ramblock dirty bitmap. As a result, a guest with host
memory backend may crash after migration.

Fix it by adding the offset of ramblock when accessing the dirty memory
bitmap of ram_list in cpu_physical_memory_sync_dirty_bitmap().

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
---
 include/exec/ram_addr.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index 73d1bea8b6..cbc797ed05 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -377,6 +377,7 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
                                                uint64_t *real_dirty_pages)
 {
     ram_addr_t addr;
+    ram_addr_t offset = rb->offset;
     unsigned long page = BIT_WORD(start >> TARGET_PAGE_BITS);
     uint64_t num_dirty = 0;
     unsigned long *dest = rb->bmap;
@@ -386,8 +387,9 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
         int k;
         int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
         unsigned long * const *src;
-        unsigned long idx = (page * BITS_PER_LONG) / DIRTY_MEMORY_BLOCK_SIZE;
-        unsigned long offset = BIT_WORD((page * BITS_PER_LONG) %
+        unsigned long word = BIT_WORD((start + offset) >> TARGET_PAGE_BITS);
+        unsigned long idx = (word * BITS_PER_LONG) / DIRTY_MEMORY_BLOCK_SIZE;
+        unsigned long offset = BIT_WORD((word * BITS_PER_LONG) %
                                         DIRTY_MEMORY_BLOCK_SIZE);
 
         rcu_read_lock();
@@ -416,7 +418,7 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
     } else {
         for (addr = 0; addr < length; addr += TARGET_PAGE_SIZE) {
             if (cpu_physical_memory_test_and_clear_dirty(
-                        start + addr,
+                        start + addr + offset,
                         TARGET_PAGE_SIZE,
                         DIRTY_MEMORY_MIGRATION)) {
                 *real_dirty_pages += 1;
-- 
2.11.0