Re: [Qemu-devel] [PATCH 06/10] scsi, file-posix: add support for persistent reservation management

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Aug 22, 2017 at 03:18:28PM +0200, Paolo Bonzini wrote:
> It is a common requirement for virtual machine to send persistent
> reservations, but this currently requires either running QEMU with
> CAP_SYS_RAWIO, or using out-of-tree patches that let an unprivileged
> QEMU bypass Linux's filter on SG_IO commands.
> 
> As an alternative mechanism, the next patches will introduce a
> privileged helper to run persistent reservation commands without
> expanding QEMU's attack surface unnecessarily.

FYI, libvirt should block this helper program as it sets up capabilities
in such a way that prevent QEMU evalating privileges via setuid binaries.

We would have to figure out a way for libvirt to run the daemon I guess.

> 
> The helper is invoked through a "pr-manager" QOM object, to which
> file-posix.c passes SG_IO requests for PERSISTENT RESERVE OUT and
> PERSISTENT RESERVE IN commands.  For example:
> 
>   $ qemu-system-x86_64
>       -device virtio-scsi \
>       -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
>       -drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
>       -device scsi-block,drive=hd
> 
> or:
> 
>   $ qemu-system-x86_64
>       -device virtio-scsi \
>       -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
>       -blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
>       -device scsi-block,drive=hd
> 
> Multiple pr-manager implementations are conceivable and possible, though
> only one is implemented right now.  For example, a pr-manager could:
> 
> - talk directly to the multipath daemon from a privileged QEMU
>   (i.e. QEMU links to libmpathpersist); this makes reservation work
>   properly with multipath, but still requires CAP_SYS_RAWIO
> 
> - use the Linux IOC_PR_* ioctls (they require CAP_SYS_ADMIN though)
> 
> - more interestingly, implement reservations directly in QEMU
>   through file system locks or a shared database (e.g. sqlite)

IIUC This last thing is essentially what libvirt already provided via
its virtlockd daemon. For SCSI disks, we have a configuration option that
tells it to run the '/lib/udev/scsi_id' program, and then acquires a
fcntl() lock on a file  in /var/lib/libvirt/lockd/scsivolumes/ whose
name is based on the value reported by scsi_id.

Regards,
Daniel
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