[OpenIndiana-discuss] ZVOL (et al) /device node access rights
Heinrich van Riel
heinrich.vanriel at gmail.com
Tue Oct 16 01:11:33 UTC 2012
I have been looking at changing from file to zvol, not for performance
A few questions on using ZVOL
1. Is it as stable as using a file for disk?
2. Is it possible that snapshots sizes could be smaller vs file? (This is
my main reason, snapshot replication.)
3. Can this be used with KVM? I am thinking about dropping vbx ose once I
have all amd systems replaced.
On Mon, Oct 15, 2012 at 11:49 AM, Andrej Javoršek <drejc at ntf.uni-lj.si>wrote:
> I'm running VB guests as normal (unprivileged) user and I have impression
> that ownership (I'm using chown on zvol's) is not always lost during
> The other part of my comment was joke on my behalf, since restarts of host
> OS are rare and often I forget to check if guests came up properly.
> But I completely agree with you that the behaviour is not favorable.
> Regards Andrej
> On Mon, Oct 15, 2012 at 4:39 PM, Jim Klimov <jimklimov at cos.ru> wrote:
> > I am not sure I understood your comment?..
> > 2012-10-15 11:14, Andrej Javoršek wrote:
> >> Hello,
> >> I have impression that it is not always necessary to chown raw zvol's.
> > It seems necessary when we need to allow a non-root user to use the
> > zvol directly, such as a backing store for his VM's virtual disk.
> > It happens occasionally on some zvols (and only when I initiate reboot
> >> and forget about it) :)
> > What happens? The need to chown?
> > Sorry for misunderstandings if any,
> > //Jim
> >> 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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