[OpenIndiana-discuss] ZVOL (et al) /device node access rights

Andrej Javoršek drejc at ntf.uni-lj.si
Mon Oct 15 07:14:39 UTC 2012

I have impression that it is not always necessary to chown raw zvol's. It
happens occasionally on some zvols (and only when I initiate reboot and
forget about it)  :)

Regards Andrej

On Sun, Oct 14, 2012 at 3:08 PM, Jim Klimov <jim at cos.ru> wrote:

> While updating the Wiki page on virtualization, Edward Ned Harvey
> wrote of, and brought to my attention, this peculiar situation:
> A VirtualBox VM can use delegated zvols as "dsk" or "rdsk" devices
> on the host, just like it can use delegated raw disks or partitions,
> likely iSCSI volumes and other block devices. According to Edward,
> block devices yield better performance than VDI files for VM disks.
> A VM can be executed by an unprivileged user, and thus the device
> node needs to be RW accessible to that non-root user (whom and why
> to trust - that's the admin's problem, OS should not limit that).
> So, the problem detected with ZVOLs (and I expect it can have a
> wider range on other devices) is that the ownership of the device
> node for a zvol is forgotten upon reboot or other pool reimport.
> That is, the node used by a VM should be chown'ed upon every VM
> startup. That's inconvenient, so to say.
> I played more with this and found that I can also set ACLs with
> /bin/chmod on device nodes, and that is even remembered across
> reboots, however with /dev/zvol/*dsk/pool/vol being a dynamically
> assigned symlink like /devices/pseudo/zfs at 0:4(,raw) there is a
> problem: the symlink and device node is created when I look at
> it (i.e. upon first "ls" or another access to the /dev/zvol/...
> object), and the device node occupies the first available number.
> The /devices filesystem seems to remember ACL entries (but not
> ownerships) across reboots only in conjunction with its object
> names, so upon each reboot (reimport) of the pool, the same
> device node name can get assigned to different zvols.
> This is not only "useless" in terms of stably providing access
> to certain devices for certain users, but also harmful as after
> a reboot an unexpected user (among those earlier trusted) can
> gain access to incorrect devices (and might even enforce that
> somehow, by being first to access the device at the correct
> moment) and cause DoS or intentional illicit access to other
> users' data.
> So here is the picture "as is". I am not sure what exactly to ask,
> so I guess it's a call for opinions on how the situation can be
> improved, in terms of remembering correct ownerships and ACLs for
> those devices (not nodes) that the rights were set for, in order
> to both increase usability and security of non-root device access.
> In the particular case of ZVOL devices, I guess attributes can
> be added to the ZVOLs that would hold the POSIX and ACL access
> rights and owner:group info (do people agree that is a worthy RFE?).
> For non-zfs devices like local disk or iscsi or USB - I am not sure
> if the problem exists the same way (not tested) or how it can be
> addressed if it exists (some config file for devfs?)
> Thanks,
> //Jim Klimov
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