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

[Juan Quintela]

Haozhong Zhang <address@hidden> wrote:
> 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>

Reviewed-by: Juan Quintela <address@hidden>

As this function is only used on migration, should I integrate it on my
next push, or do you want to pull it, Paolo?

Later, Juan.


> ---
>  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;