Re: [Qemu-devel] [PATCH v7 0/6] file descriptor passing using fd sets

[Stefan Hajnoczi]

On Tue, Aug 7, 2012 at 4:58 PM, Corey Bryant <address@hidden> wrote:
> libvirt's sVirt security driver provides SELinux MAC isolation for
> Qemu guest processes and their corresponding image files.  In other
> words, sVirt uses SELinux to prevent a QEMU process from opening
> files that do not belong to it.
>
> sVirt provides this support by labeling guests and resources with
> security labels that are stored in file system extended attributes.
> Some file systems, such as NFS, do not support the extended
> attribute security namespace, and therefore cannot support sVirt
> isolation.
>
> A solution to this problem is to provide fd passing support, where
> libvirt opens files and passes file descriptors to QEMU.  This,
> along with SELinux policy to prevent QEMU from opening files, can
> provide image file isolation for NFS files stored on the same NFS
> mount.
>
> This patch series adds the add-fd, remove-fd, and query-fdsets
> QMP monitor commands, which allow file descriptors to be passed
> via SCM_RIGHTS, and assigned to specified fd sets.  This allows
> fd sets to be created per file with fds having, for example,
> different access rights.  When QEMU needs to reopen a file with
> different access rights, it can search for a matching fd in the
> fd set.  Fd sets also allow for easy tracking of fds per file,
> helping to prevent fd leaks.
>
> Support is also added to the block layer to allow QEMU to dup an
> fd from an fdset when the filename is of the /dev/fdset/nnn format,
> where nnn is the fd set ID.
>
> No new SELinux policy is required to prevent open of NFS files
> (files with type nfs_t).  The virt_use_nfs boolean type simply
> needs to be set to false, and open will be prevented (and dup will
> be allowed).  For example:
>
>     # setsebool virt_use_nfs 0
>     # getsebool virt_use_nfs
>     virt_use_nfs --> off
>
> Corey Bryant (6):
>   qemu-char: Add MSG_CMSG_CLOEXEC flag to recvmsg
>   qapi: Introduce add-fd, remove-fd, query-fdsets
>   monitor: Clean up fd sets on monitor disconnect
>   block: Convert open calls to qemu_open
>   block: Convert close calls to qemu_close
>   block: Enable qemu_open/close to work with fd sets
>
>  block/raw-posix.c |   42 ++++-----
>  block/raw-win32.c |    6 +-
>  block/vdi.c       |    5 +-
>  block/vmdk.c      |   25 +++--
>  block/vpc.c       |    4 +-
>  block/vvfat.c     |   16 ++--
>  cutils.c          |    5 +
>  monitor.c         |  273 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  monitor.h         |    5 +
>  osdep.c           |  117 +++++++++++++++++++++++
>  qapi-schema.json  |  110 +++++++++++++++++++++
>  qemu-char.c       |   12 ++-
>  qemu-common.h     |    2 +
>  qemu-tool.c       |   20 ++++
>  qerror.c          |    4 +
>  qerror.h          |    3 +
>  qmp-commands.hx   |  131 +++++++++++++++++++++++++
>  savevm.c          |    4 +-
>  18 files changed, 730 insertions(+), 54 deletions(-)

Are there tests for this feature?  Do you have test scripts used
during development?

Here's what I've gathered:

Applications use add-fd to add file descriptors to fd sets.  An fd set
contains one or more file descriptors, each with different access
modes (O_RDONLY, O_RDWR, O_WRONLY).  File descriptors can be retrieved
from the fd set and are matched by their access modes.  This allows
QEMU to reopen files with different access modes.

File descriptors stay in their fd set until explicitly removed by the
remove-fd command or when all monitor clients have disconnected.  This
ensures that file descriptors are not leaked after a monitor client
crashes.  Automatic removal on monitor close is postponed until all
duped fds have been fd - this means QEMU can still reopen an in-use fd
after a client disconnects.

Does this sound right?

Please do the QEMU coding style naming of MonFdset/MonFdsetFd mentioned in v6.

Stefan