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

On 08/08/2012 11:58 AM, Stefan Hajnoczi wrote:
On Wed, Aug 8, 2012 at 3:54 PM, Corey Bryant <coreyb linux vnet ibm com> wrote:

On 08/08/2012 09:04 AM, Stefan Hajnoczi wrote:

On Tue, Aug 7, 2012 at 4:58 PM, Corey Bryant <coreyb linux vnet ibm com>

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

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

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?

Yes I have some C code that I've been using for testing.  I can clean it up
and provide it if you'd like.

That would be very useful.  tests/ has test cases.  For the block
layer tests/qemu-iotests/ is especially relevant, that's where a lot
of the test cases go.  If you look at test case 030 you'll see how a
Python script interacts with QMP to test image streaming -
unfortunately I think Python doesn't natively support SCM_RIGHTS.  But
a test script would be very useful so it can be used as a regression
test in the future.

Sure I'll take a look. Hopefully a C test is ok if I can't use SCM_RIGHTS in Python.

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

I assume you mean "... until all duped fds have been *closed* - ..."

Yes, my typo :)

Great, then your understanding of how this works is correct. :)

after a client disconnects.

Does this sound right?

Yes, exactly.

I should point out there is an issue that needs to be cleaned up in the
future.  There are short windows of time where refcount can get to zero
while an image file is in use.  This is because the file is being reopened.
For example, I've noticed this occurs when format= is not specified on the
device_add command and the file is probed, and when mouting/unmounting a
file system.  Hopefully this can be treated as a follow-up issue.

The block layer doesn't treat this as a "reopen" today.  Supriya
Kannery has a patch series for bdrv_reopen() which would also need to
be integrated with fd sets to ensure the refcount doesn't hit 0 and
cause a cleanup.

Great, Supriya's patches sound like what is needed. Also, I noticed that I'm missing a patch in my series. I need to make sure that /dev/fdset/nnn is not detected as a floppy drive (/dev/fdx). That was causing a close/open.


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