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

[Paolo Bonzini]

On 28/06/2017 10:37, Haozhong Zhang 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>
> ---
> 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;
> 

Acked-by: Paolo Bonzini <address@hidden>

Juan, please take care of this yourself!  Thanks,

Paolo