Re: [PATCH v3 15/21] migration/block-dirty-bitmap: relax error handling in incoming part

[Eric Blake <eblake@redhat.com>]

On 7/24/20 3:43 AM, Vladimir Sementsov-Ogievskiy wrote:
> Bitmaps data is not critical, and we should not fail the migration (or
> use postcopy recovering) because of dirty-bitmaps migration failure.
> Instead we should just lose unfinished bitmaps.
> 
> Still we have to report io stream violation errors, as they affect the
> whole migration stream.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   migration/block-dirty-bitmap.c | 152 +++++++++++++++++++++++++--------
>   1 file changed, 117 insertions(+), 35 deletions(-)
> 

> @@ -650,15 +695,32 @@ static int dirty_bitmap_load_bits(QEMUFile *f, DBMLoadState *s)
>   
>       if (s->flags & DIRTY_BITMAP_MIG_FLAG_ZEROES) {
>           trace_dirty_bitmap_load_bits_zeroes();
> -        bdrv_dirty_bitmap_deserialize_zeroes(s->bitmap, first_byte, nr_bytes,
> -                                             false);
> +        if (!s->cancelled) {
> +            bdrv_dirty_bitmap_deserialize_zeroes(s->bitmap, first_byte,
> +                                                 nr_bytes, false);
> +        }
>       } else {
>           size_t ret;
>           uint8_t *buf;
>           uint64_t buf_size = qemu_get_be64(f);

Pre-existing, but if I understand, we are reading a value from the 
migration stream...

> -        uint64_t needed_size =
> -            bdrv_dirty_bitmap_serialization_size(s->bitmap,
> -                                                 first_byte, nr_bytes);
> +        uint64_t needed_size;
> +
> +        buf = g_malloc(buf_size);
> +        ret = qemu_get_buffer(f, buf, buf_size);

...and using it to malloc memory.  Is that a potential risk of a 
malicious stream causing us to allocate too much memory in relation to 
the guest's normal size?  If so, fixing that should be done separately.

I'm not a migration expert, but the patch looks reasonable to me.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org