Re: [RFC 3/3] ima: make the integrity inode cache per namespace

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On Mon, Nov 29, 2021 at 12:04:29PM -0500, Stefan Berger wrote:
> 
> On 11/29/21 11:16, Christian Brauner wrote:
> > On Mon, Nov 29, 2021 at 09:35:39AM -0600, Serge Hallyn wrote:
> > > On Mon, Nov 29, 2021 at 09:46:55AM -0500, James Bottomley wrote:
> > > > On Mon, 2021-11-29 at 15:22 +0100, Christian Brauner wrote:
> > > > > On Mon, Nov 29, 2021 at 09:10:29AM -0500, James Bottomley wrote:
> > > > > 
> > I kept thinking about this question while I was out running and while I
> > admittedly have reacted poorly to CLONE_NEWIMA patches before it feels
> > to me that this is the right approach after all. Making it part of
> > userns at least in this form isn't clean.
> > 
> > I think attaching a uuid to a userns alone for the sake of IMA is wrong.
> > Additionally, I think a uuid only for the userns is too limited. This is
> > similar to the problem of the audit container id. If we have some sort
> > of uuid for ima it will automatically evolve into something like a
> > container id (I'm not even arguing that this is necessarily wrong.).
> > We also have the issue that we then have the container audit id thing -
> > if this ever lands and the ima userns uuid. All that makes it quite
> > messy.
> > 
> > I think CLONE_NEWIMA is ultimately nicer and allows the implementation
> > to be decoupled from the userns and self-contained as possible. This
> > also means that ima ns works for privileged containers which sure is a
> > valid use-case.
> 
> The thing is that piggy backing on the user namespace at least allows us to
> 'always see' where IMA's keyring is (across setns()). If we were using an
> independent IMA namespace how would we guarantee that the user sees the
> keyring for IMA appraisal? We would at least have to take a reference (as in
> get_user_ns()) to the user namespace when the IMA namespace is created so
> that it at least the association of IMA namespace to user namespace remains
> a constant and the keyring IMA is using (and are held by the user namespace)

I don't think that this needs to be any different from other namespaces
that have an owning userns.

> is also constant across setns()'s. Otherwise it is possible that the user
> could do setns() to a different user namespace and be confused about the
> keys IMA is using. So at least by piggy backing we have them together. The
> aspect here may be 'usability'.

I mean, we do already have a dependence between pid namespaces and user
namespace etc., i.e. before you can join a pidns as an unpriv user you
need to join the userns. I think we can easily introduce a dependency
there. (Note also that a while back I extended setns() to take a pidfd
as an argument and you can now specify setns(pidfd, CLONE_NEWUSER |
CLONE_NEWPID | CLONE_NEWIMA).)

It's even worse in a sense since we can joing CLONE_NEWUSER in different
order depending whether we're privileging or devprivileging ourselves
(see the messy logic in nsenter and new*idmap for example). So there's
precedence for requiring dependencies between namespaces during setns().

> 
> I am somewhat sold on the USER namespace piggy backing thing... as suggested
> by James.
> 
> 
> > It will also make securityfs namespacing easier as it can be a keyed
> > super where the key is the ima ns (similar to how we deal with e.g.
> > mqueue).
> 
> Yes, mqueue is where I got the (API usage) idea from how to switch out the
> reference to the user namespace needed for the 'keyed' approach.
> 
> I will massage my 20 patch-series a bit more and then post it (for
> reference....). It does have 'somewhat dramatic' things in there that stem
> from virtualizing securityfs for example and IMA being a dependent of the
> user namespace and taking one more reference to the user namespace (it is a
> dependent of) and then the user namespace not being able to easily delete:
> 
> It's this here to help with an early tear-down to decrease the reference
> counter.
> 
> - https://github.com/stefanberger/linux-ima-namespaces/commit/1a5d7f2598764ca6f1a8c5a391672543fef83f2c
> 
> - https://github.com/stefanberger/linux-ima-namespaces/commit/d246f501f977e64333ecbd8bb79994e23b552b9b
> 
> - https://github.com/stefanberger/linux-ima-namespaces/commit/3b82058936862d7623b3a06bc1749d5efc018ab1#diff-99458ca9139231ac3811dbb0c0fced442c46c7cfdb94e86e4553fc0329d3a79bR647-R651
> 
> The teardown variable makes this a controlled process but ... is it
> acceptable?

I'll try to take a look later this week if that's ok.



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