[Qemu-devel] [PATCH v2] exec: fix access to ram_list.dirty_memory when sync dirty bitmap

[Haozhong Zhang]

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 <address@hidden>
Signed-off-by: Haozhong Zhang <address@hidden>
---
Changes in v2:
 * Avoid shadowing variable 'offset'. (Paolo)
---
 include/exec/ram_addr.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index 73d1bea8b6..c04f4f67f6 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -386,8 +386,9 @@ 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 + rb->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();
@@ -414,9 +415,11 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock 
*rb,
 
         rcu_read_unlock();
     } else {
+        ram_addr_t offset = rb->offset;
+
         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