Re: [Qemu-devel] Race condition in VMDK (QCOW*) formats.

[Fabrice Bellard <fabrice@bellard.org>]

Well, it was never said that the QCOW* code was safe if you interrupted 
QEMU at some point.

But I agree that it could be safer to write the sector first and update 
the links after. It could be interesting to analyze the QCOW2 snapshots 
handling too (what if QEMU is stopped during the creation of a snapshot ?).

Regards,

Fabrice.

Igor Lvovsky wrote:
> 
> 
>  Hi all,
> 
> I have doubt about the race condition during the *write operation on 
> snapshot*.
> 
> I think the problem exists in VMDK and QCOW* formats (I didn't checked 
> the others).
> 
>  
> 
> The example from the block_vmdk.c.
> 
>  
> 
> static int vmdk_write(BlockDriverState *bs, int64_t sector_num,
> 
>                      const uint8_t *buf, int nb_sectors)
> 
> {
> 
>     BDRVVmdkState *s = bs->opaque;
> 
>     int ret, index_in_cluster, n;
> 
>     uint64_t cluster_offset;
> 
>  
> 
>     while (nb_sectors > 0) {
> 
>         index_in_cluster = sector_num & (s->cluster_sectors - 1);
> 
>         n = s->cluster_sectors - index_in_cluster;
> 
>         if (n > nb_sectors)
> 
>             n = nb_sectors;
> 
>         cluster_offset = get_cluster_offset(bs, sector_num << 9, 1);
> 
>         if (!cluster_offset)
> 
>             return -1;
> 
>         lseek(s->fd, cluster_offset + index_in_cluster * 512, SEEK_SET);
> 
>         ret = write(s->fd, buf, n * 512);
> 
>         if (ret != n * 512)
> 
>             return -1;
> 
>         nb_sectors -= n;
> 
>         sector_num += n;
> 
>         buf += n * 512;
> 
>     }
> 
>     return 0;
> 
> }
> 
>  
> 
> The /get_cluster_offset(…)/ routine update the L2 table of the metadata 
> and return the /cluster_offset. /
> 
> After that the /vmdk_write(…)/ routine/ /actually write the grain at 
> right place.
> 
> So, we have timing hole here.
> 
>  
> 
> Assume, VM that perform write operation will be destroyed at this moment.
> 
> So, we have corrupted image (with updated L2 table, but without the 
> grain itself).
> 
>  
> 
>             Regards,
> 
>                         Igor Lvovsky
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
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> 
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