[libvirt] [PATCH v2 4/4] security: don't relabel chardev source if virtlogd is used as stdio handler

Thu Jun 15 14:40:49 UTC 2017

On Thu, Jun 15, 2017 at 07:57:18AM -0400, John Ferlan wrote:
> 
> 
> On 06/15/2017 03:11 AM, Pavel Hrdina wrote:
> > On Tue, Jun 13, 2017 at 08:00:41PM -0400, John Ferlan wrote:
> >>
> >>
> >> On 05/29/2017 10:31 AM, Pavel Hrdina wrote:
> >>> In the case that virtlogd is used as stdio handler we pass to QEMU
> >>> only FD to a PIPE connected to virtlogd instead of the file itself.
> >>>
> >>> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1430988
> >>>
> >>> Signed-off-by: Pavel Hrdina <phrdina at redhat.com>
> >>> ---
> >>>
> >>> Notes:
> >>>     new in v2
> >>>
> >>>  src/lxc/lxc_process.c            |  6 ++---
> >>>  src/qemu/qemu_security.c         |  9 +++++--
> >>>  src/security/security_apparmor.c |  7 ++++--
> >>>  src/security/security_dac.c      | 54 +++++++++++++++++++++++++++++++---------
> >>>  src/security/security_driver.h   |  6 +++--
> >>>  src/security/security_manager.c  | 12 ++++++---
> >>>  src/security/security_manager.h  |  6 +++--
> >>>  src/security/security_nop.c      |  6 +++--
> >>>  src/security/security_selinux.c  | 53 ++++++++++++++++++++++++++++++---------
> >>>  src/security/security_stack.c    | 12 ++++++---
> >>>  tests/securityselinuxlabeltest.c |  2 +-
> >>>  11 files changed, 127 insertions(+), 46 deletions(-)
> >>>
> >>
> >> Why is it a (!chr_seclabel && chardevStdioLogd)?  More to the point why
> >> is (!chr_seclabel) even matter?
> > 
> > If you configure the label we shouldn't ignore it in some cases, that's
> > just wrong.  If the label is set for the char device we will configure
> > it every time even if it will fail to start the guest, it's a
> > responsibility of the user to configure proper label if it is provided.
> > 
> 
> When email doesn't convey the question... Ugh...  I'm also trying to
> speed learn an area of the code and review at the same time.
> 
> If I go back to commit id 'f8b08d0e' where the original implementation
> to add labels for chardevs was done (but has been modified for patch 1
> to change where the label is stored), I get the impression that a label
> should be added either from something specifically supplied for the
> <chardev> or to use the per domain one:
> 
> "The source element may contain an optional seclabel to override the way
> that labelling is done on the socket path. If this element is not
> present, the security label is inherited from the per-domain setting."
> 
> So if I look at the condition "(!chr_seclabel && chardevStdioLogd)"
> added by this patch to decide whether or not to supply the label, I'm
> left with the impression that if for this particular chardev a label
> doesn't exist *and* the configuration option chardevStdioLogd is true,
> then we're going to return happy status *and* not inherit the per-domain
> setting.
> 
> So the bug is then that applying the default domain label for a chardev
> configured to use a stdio handle is incorrect?  Perhaps I didn't get
> that from reading bz or the patch.

Yes, that's the bug.  If virtlogd is used to handle stdio for char
devices we shouldn't relabel the @path to the default labels, the
@path doesn't have to be accessible by the qemu process, it has to be
accessible by the virtlogd process.

> >> IIUC, whether or not someone set the label for the chardev, for this
> >> particular issue/config where the chardev has a <log file=$path>, we
> >> don't want to set (or restore) the label. I feel like I'm missing
> >> something subtle. Maybe a bit more explanation of the adjustment would
> >> help me...
> > 
> > This is not for the <log file=$path/> but for the <source path=$path/>.
> > We don't relabel $path for <log file=$path/> at all.
> > 
> 
> hmm.. ah, right... I kept scrolling back and forth in the bz and the
> docs, but missed this in the bz:
> 
> 3) Check the virtlogd.log:
> error : virRotatingFileWriterEntryNew:113 : Unable to open file:
> /var/log/libvirt/qemu/log: Permission denied
> 
> I guess I got lost in the power of suggestion of reading the docs
> regarding the "optional log file" that can be associated paragraph and
> trying to learn on the fly so that you at least get a review in a
> somewhat timely manner ;-)
> 
> >> Wouldn't these changes end up selecting "any" chardev if
> >> chardevStdioLogd ended up being set regardless of whether they were
> >> actually using the log file?
> > 
> > I don't know what you mean by this sentence?
> > 
> 
> Well let's see, chardevStdioLogd is set to true when meeting the two
> conditions a qemu.conf global "cfg->stdioLogd" && a per domain or
> emulator image capability "QEMU_CAPS_CHARDEV_FILE_APPEND".
> 
> So, conceivably chardevStdioLogd could be true for *any* domain as long
> as those conditions are met, right?

Yes, the two conditions are checked while starting a new domain in
qemuProcessPrepareDomain() and stored in the private date of that
domain.

> If you have a domain that has chardev's which are not configured to use
> the stdio handler, then the chardevStdioLogd could still be true, right?

No, if the @chardevStdioLogd is true all char devices for that domain
will use virtlogd.

The only issue I've just found out is that the code path for chardev
hot-plug isn't updated to use virtlogd when it should be so for
hot-plugged char devices we pass the path directly to QEMU.

With this patch applied the hot-plug fails if virtlogd is used because
we don't relabel the path but we pass it directly to QEMU, this needs to
be fixed to not introduce a regression, sigh.

Pavel

> If that's the case and the chardev doesn't have a label associated, then
> we just return happy status and we do not inherit the per domain
> setting. Wouldn't that be incorrect?
> 
> My concern is more we're making a change in a (mostly) common set of
> functions for a (very) specific problem.
> 
> 
> >> As an aside, I think there's an "oddity" when it comes to the Restore,
> >> but I'm not sure how to put it into words exactly. If a guest is running
> >> code prior to this set of changes, would it have successfully run a Set?
> >> If so, then after applying this change and restarting, the label
> >> wouldn't be reset, right?  What happens at guest shutdown - does the
> >> label not get unset now?  Of course this is all "interaction" with
> >> virtlogd restart that really throws a monkey wrench into things.
> > 
> > No, that's not correct.  The @chardevStdioLogd is stored in the status
> > XML (the one stored in /var/run/libvirt/qemu/$domain_name.xml).  So when
> > the libvirtd is stopped and started with this patch applied the status
> > XML doesn't have the @chardevStdioLogd stored in it so it will be false
> > and we will reset the label.  The @chardevStdioLogd is updated only when
> > the domain is started and we will store the value in the status XML
> > only with new libvirtd and only in that case we will not set/restore
> > the label.
> > 
> 
> hmmm.. Reading the bz indicates the 'virtlogd' daemon restarting... This
> is where all this gets a bit "odd" for me. Like I said it was a weird
> thing to even try and explain, but I think you talked me off the ledge
> of concern.
> 
> John
> 
> >> Also, why is the Smartcard chardev handling not using this
> > 
> > The smartcard doesn't ever use virtlogd as stdio handler.
> > 
> > Pavel
> > 
> 
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