Re: Fwd: [SMACK]Problem with user naespace

[Serge E. Hallyn]

Quoting Jacek Pielaszkiewicz (j.pielaszkie@xxxxxxxxxxx):
> My comments below.
> 
> Best regards
> 
> 
> Jacek Pielaszkiewicz
> Samsung R&D Institute Poland
> Samsung Electronics
> Email: j.pielaszkie@xxxxxxxxxxx
> 
> 
> > -----Original Message-----
> > From: Serge E. Hallyn [mailto:serge@xxxxxxxxxx]
> > Sent: Saturday, March 29, 2014 5:16 PM
> > To: Jacek Pielaszkiewicz
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: Fwd: [SMACK]Problem with user naespace
> > 
> > Quoting Jacek Pielaszkiewicz (j.pielaszkie@xxxxxxxxxxx):
> > >
> > > Hi
> > >
> > >     I have problem with starting lxc containers when SMACK is enabled
> > > on the host. The issue appears when systemd try start user session in
> > > the container. In such case systemd reports error that has not
> > > permissions to set SMACK label. In my test configuration lxc
> > container
> > > has full separation (all namespaces are enabled - including user
> > namespace).
> > >     The issue is common. The problem is due to lack of permissions of
> > > the task to write into /proc/self/sttr/current file even the task has
> > > active CAP_MAC_ADMIN capability. Regarding to may tests the issue is
> > > connected to user namespace.
> > >
> > >     I have prepared patch (see below). The patch was tested and
> > > created on kernel 3.10.
> > >
> > > From 1d42d88fccafb7a9fa279588bc827163484a7852 Mon Sep 17 00:00:00
> > 2001
> > > From: Jacek Pielaszkiewicz <j.pielaszkie@xxxxxxxxxxx>
> > > Date: Mon, 24 Mar 2014 14:11:58 +0100
> > > Subject: [PATCH] [PATCH] Enable user namespace in SMACK
> > >
> > > SMACK: Enable user namespace
> > >
> > > - Bug/Issue:
> > > The issue has been found when we tried to setup lxc container.
> > > We tried to setup the container with active all linux namespaces
> > > (including user namespace). On the host as LSM was enabled SMACK.
> > >
> > > We have found that "systemd" was not able to setup user sessiondue to
> > > lack of permissions to write into /proc/self/attr/currentfile.
> > >
> > > We have found general issue that any privileged process with enabled
> > > CAP_MAC_ADMIN cannot write into /proc/self/attr/currentfile.
> > >
> > > - Cause:
> > > SMACK to check the task capabilities uses capable().
> > >
> > > - Solution:
> > > The fix replaces usage capable() by ns_capable().
> > >
> > > Becuase SMACK uses globally defined rules usage ns_capable() was
> > > limited to places where requested by task operation are connected to
> > > himself. All operation that modify global SMACK rules remain
> > unchanged
> > > - SMACK still to test capabilities calls capable(). It means that
> > > operation on smackfs remain unchanged - operation are allowed only by
> > > host.
> > >
> > > Change-Id: I0bb28fa02943dd7ccb38dda2c8a37dcce2d551b2
> > > Signed-off-by: Jacek Pielaszkiewicz <j.pielaszkie@xxxxxxxxxxx>
> > > ---
> > >  security/smack/smack.h        | 13 +++++++++++++
> > >  security/smack/smack_access.c |  2 +-
> > >  security/smack/smack_lsm.c    | 16 ++++++++--------
> > >  3 files changed, 22 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/security/smack/smack.h b/security/smack/smack.h index
> > > d072fd3..9f9d5e7 100644
> > > --- a/security/smack/smack.h
> > > +++ b/security/smack/smack.h
> > > @@ -319,6 +319,19 @@ static inline int smack_privileged(int cap)  }
> > >
> > >  /*
> > > + * Is the task privileged and allowed to privileged
> > > + * by the onlycap rule in user namespace.
> > > + */
> > > +static inline int smack_privileged_ns(int cap) {
> > > +       if (!ns_capable(current_user_ns(), cap))
> > > +               return 0;
> > 
> > As I responded on lxc-devel (sorry I had apparently missed this
> > original mail),
> > 
> > This is an often seen, but very wrong, idiom.  This check means
> > nothing, because any unprivileged user can create a new userns and pass
> > this check.
> 
> I spent two days thinking how to fix the issue. I also discussed the issue
> with
> my colleagues. The issue seems not to be trivial. 
> Generally the SMACK is not ready to support namespaces and definitely 
> it was designed to work on host only (in root namespace only). 
> Of course you are right - patch like my cause that any unprivileged process 
> in his own user namespace can set any label, what from practical 
> point of view means that "SMACK" is disabled.
> 
> >From my perspective the issue is caused by changed that was implemented 
> in kernel 3.8. From this version and later any process can creates own 
> user namespace. In the older kernels it was limited to privileged processes.
> 
> Perhaps this change should be rollback?

?  The problem is that admins in a non-init userns cannot be
smack_capable() so you can't create an unprivileged container.
Right?  (Please correct me if I'm wrong).  So why would we
revert the ability for unprivileged users to unshare(CLONE_NEWUSER)?
If you don't want to use unprivileged containers, don't use
unprivileged containers.

Have you talked to Casey about this?  (cc:d)

As I said, it'll just take a bit more work to check for privilege over
the right things.

-serge
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