Re: [Qemu-devel] [PATCH 0/7] Add checks for corruption in the snapshot table

[Kevin Wolf]

Am 01.03.2018 um 17:27 hat Alberto Garcia geschrieben:
> Hey ho!
> 
> As we have already discussed in the mailing list, the offset and size
> values of snapshots' L1 tables are almost never validated in the QEMU
> code.
> 
> One way to deal with this is to validate them when the image is being
> opened (in qcow2_read_snapshots()) and return an error if there's
> anything wrong. However this would render the image unusable because
> we wouldn't be able to try to recover data using the active table as
> we can do now.
> 
> Another possibility is to refuse opening the image but add a way to
> fix it using 'qemu-img check'. But this would be a destructive
> operation, and knowing that the image is already corrupted we can't
> guarantee that we wouldn't be corrupting it even more.
> 
> So although having a way to repair this kind of corruption would be
> nice, we should still allow the user to try to open the image and read
> data from it without requiring a potentially destructive operation
> first.

Generally speaking, there is also always the option to refuse opening a
corrupted image read-write, but to allow opening it read-only. This
would probably still require some checks on use, though (or maybe just
hiding all snapshots).

Kevin

> So this series simply validates the snapshots' L1 tables in the places
> where they are actually used. We already have a function to do this
> kind of checks so we only need to call it.
> 
> Regards,
> 
> Berto
> 
> Alberto Garcia (7):
>   qcow2: Generalize validate_table_offset() into qcow2_validate_table()
>   qcow2: Check L1 table offset in qcow2_snapshot_load_tmp()
>   qcow2: Check L1 table parameters in qcow2_expand_zero_clusters()
>   qcow2: Check snapshot L1 tables in qcow2_check_metadata_overlap()
>   qcow2: Check snapshot L1 table in qcow2_snapshot_goto()
>   qcow2: Check snapshot L1 table in qcow2_snapshot_delete()
>   qcow2: Make qemu-img check detect corrupted L1 tables in snapshots
> 
>  block/qcow2-cluster.c      | 20 +++++++-----
>  block/qcow2-refcount.c     | 24 ++++++++++++++-
>  block/qcow2-snapshot.c     | 21 +++++++++++--
>  block/qcow2.c              | 77 
> ++++++++++++++++++----------------------------
>  block/qcow2.h              | 10 +++---
>  tests/qemu-iotests/080     | 22 ++++++++++++-
>  tests/qemu-iotests/080.out | 54 ++++++++++++++++++++++++++------
>  7 files changed, 155 insertions(+), 73 deletions(-)
> 
> -- 
> 2.11.0
>