Re: [PATCH] libxl: set permissions for xs frontend entry pointing to xs backend

["Roger Pau Monné" <roger.pau@citrix.com>]

On 10/09/13 17:12, Ian Jackson wrote:
> Roger Pau Monne writes ("[PATCH] libxl: set permissions for xs frontend entry pointing to xs backend"):
>> libxl doesn't currently set the permissions of entries like:
>>
>> /local/domain/<domid>/device/<dev_type>/<devid>/backend
>>
>> This allows the guest to change this xenstore entries to point to a
>> different backend path, or to malicious xenstore path forged by the
>> guest itself. libxl currently relies on this path being valid in order
>> to perform the unplug of devices in libxl__devices_destroy, so we
>> should prevent the guest from modifying this xenstore entry.
> 
> Is it sufficient to set the permissions on "backend" - does that
> prevent the guest deleting the whole subtree ?

No, the guest can still delete the whole subtree, but it can not
recreate it (because the parent directory
/local/domain/<domid>/device/<dev_type>/ is not writeable by the guest).

> Really it would be better to make the unplug not depend on this path.
> 
> This is a security issue, so CCing security@.  It appears to have
> been discovered in public on xen-devel, so shouldn't be embargoed.
> 
> Ian.
>