[libvirt] [PATCH] snapshot: conf: Fix NULL dereference when <driver> element is empty

[Daniel P. Berrange]

On Wed, Nov 13, 2013 at 12:07:43PM +0100, Peter Krempa wrote:
> Consider the following valid snapshot XML as the <driver> element is
> allowed to be empty in the domainsnapshot.rng schema:
> 
> $ cat snap.xml
> <domainsnapshot>
>   <disks>
>     <disk name='vda' snapshot='external'>
>       <source file='/tmp/foo'/>
>       <driver/>
>     </disk>
>   </disks>
> </domainsnapshot>
> 
> produces the following error:
> 
> $ virsh snapshot-create domain snap.xml
> error: internal error: unknown disk snapshot driver '(null)'
> 
> The driver type is parsed as NULL from the XML as the attribute is not
> present and then directly used to produce the error message.
> 
> With this patch the attempt to parse the driver type is skipped if not
> present to avoid changing the schema to forbid the empty driver element.
> ---
>  src/conf/snapshot_conf.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/src/conf/snapshot_conf.c b/src/conf/snapshot_conf.c
> index d8910c9..418987b 100644
> --- a/src/conf/snapshot_conf.c
> +++ b/src/conf/snapshot_conf.c
> @@ -175,15 +175,17 @@ virDomainSnapshotDiskDefParseXML(xmlNodePtr node,
>          } else if (!def->format &&
>                     xmlStrEqual(cur->name, BAD_CAST "driver")) {
>              char *driver = virXMLPropString(cur, "type");
> -            def->format = virStorageFileFormatTypeFromString(driver);
> -            if (def->format <= 0) {
> -                virReportError(VIR_ERR_INTERNAL_ERROR,
> -                               _("unknown disk snapshot driver '%s'"),
> -                               driver);
> +            if (driver) {
> +                def->format = virStorageFileFormatTypeFromString(driver);
> +                if (def->format <= 0) {
> +                    virReportError(VIR_ERR_INTERNAL_ERROR,
> +                                   _("unknown disk snapshot driver '%s'"),
> +                                   driver);
> +                    VIR_FREE(driver);
> +                    goto cleanup;
> +                }
>                  VIR_FREE(driver);
> -                goto cleanup;
>              }
> -            VIR_FREE(driver);
>          }
>      }

So IIUC, this allows

  <driver type="qemu"/>
  <driver type="qemu" format="qcow2" />

but forbids

  <driver format="qcow2" />

The domain XML parser, however, allows all 3 and 'type' will be auto
determined if omitted. IMHO it would be desirable to be consistent
between the parsers

Daniel
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