That's awesome and now it's got me thinking about other classes/permissions that we could implement. Can cil macros can be referenced in .te/.if files?
On Tue, Sep 11, 2018 at 2:27 PM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
On 09/11/2018 02:49 PM, Ted Toth wrote:
> Yes I too noticed the translate permission but couldn't find any info
> related to it intended purpose. Regarding CIL unfortunately I have zero
> experience with it but I've installed the compiler and started reading
> through https://github.com/SELinuxProject/cil/wiki (any other pointers
> to useful info would be appreciated). I have written lots of policy
> would it be possible to add a class/permissions/mlsconstraints in an
> old-fashion policy module?
The older binary modules didn't support those kinds of statements
outside of the base module. Try this:
$ cat > mcstrans.cil <<EOF
; define a mcstrans class with one permission color_use
(class mcstrans (color_use))
; allow all domains mcstrans color_use permission to themselves
(allow domain self (mcstrans (color_use)))
; only allow mcstrans color_use permission when h1 dominates h2
(mlsconstrain (mcstrans (color_use)) (dom h1 h2))
; append the new mcstrans class to the end after all others
(classorder (unordered mcstrans))
EOF
$ sudo semodule -i mcstrans.cil
Then try performing permission checks with "mcstrans" as your class and
"color_use" as your permission, between a domain and itself, with
different levels.
>
> On Tue, Sep 11, 2018 at 1:27 PM Stephen Smalley <sds@xxxxxxxxxxxxx
> <mailto:sds@xxxxxxxxxxxxx>> wrote:
>
> On 09/11/2018 10:41 AM, Stephen Smalley wrote:
> > On 09/10/2018 06:30 PM, Ted Toth wrote:
> >> mcstrans mcscolor.c also uses the same logic I'd been using to
> check
> >> dominance so this too will no longer function as expected on
> el7. Do
> >> you any suggestions for doing a 'generic' (one not tied to a
> specific
> >> resource class) dominance check in lieu of context contains?
> >
> > You should probably define your own permission with its own
> constraint
> > to avoid depending on the base policy's particular constraint
> > definitions. Certainly for your own code. For mcstrans, mcscolor
> > probably ought to be switched to using at least a separate
> permission in
> > the context class if not its own class to avoid overloading the
> meaning
> > with pam_selinux's usage (or vice versa, but likely harder to change
> > pam_selinux at this point).
> >
> > It is possible to define an entirely new class, its permissions,
> and its
> > mls constraints via a CIL module IIUC, without needing to change the
> > base policy.
> >
> > I don't think you can add a permission to an existing class via a
> CIL
> > module currently, unfortunately, so you can't just extend the
> context
> > class without modifying the base policy. So it may be easier to
> define
> > an entirely new class.
> >
> > The class and permission ought to be specific to the usage. For
> > example, mcstrans could have its own class (mcstrans) with its own
> > permissions (e.g. color_match or color_use or ...) that abstract
> away
> > the logical check being performed. Dominance checks performed for
> > different reasons ought to use different permissions so that one can
> > distinguish what TE pairs are allowed them.
> >
> > Your code could likewise define and use its own class and permission.
> >
> > Does that make sense?
>
> BTW, I noticed there is another permission ("translate") defined in the
> context class and its constraint is ((h1 dom h2) or (t1 ==
> mlstranslate)). I would have guessed that it was intended as a
> front-end service check over what processes could request context
> translations from mcstrans or what contexts they could translate, but I
> don't see it being used in mcstrans anywhere. Is this a legacy thing
> from early setransd/mcstransd days? There is a TODO comment in
> mcstrans
> process_request() that suggests there was an intent to perform a
> dominance check between the requester context and the specified
> context,
> but that's not implemented. Appears to be allowed in current policy
> for
> all domains to the setrans_t domain itself.
>
> >
> >>
> >> Ted
> >>
> >> On Mon, Sep 10, 2018 at 1:19 PM Ted Toth <txtoth@xxxxxxxxx
> <mailto:txtoth@xxxxxxxxx>
> >> <mailto:txtoth@xxxxxxxxx <mailto:txtoth@xxxxxxxxx>>> wrote:
> >>
> >> Understood, thanks.
> >>
> >> On Mon, Sep 10, 2018 at 12:46 PM Stephen Smalley
> <sds@xxxxxxxxxxxxx <mailto:sds@xxxxxxxxxxxxx>
> >> <mailto:sds@xxxxxxxxxxxxx <mailto:sds@xxxxxxxxxxxxx>>> wrote:
> >>
> >> On 09/10/2018 01:13 PM, Ted Toth wrote:
> >> > We currently have code running on el6 that does a MLS
> >> dominance check by
> >> > calling security_compute_av_raw with the security
> object class
> >> > SECCLASS_CONTEXT with permission CONTEXT__CONTAINS as
> you can
> >> see in the
> >> > python code below. When I run this code on el6 s1
> dominates
> >> s0 however
> >> > when I run the same code on el7 s1 does not dominate
> s0. On
> >> both systems
> >> > the file read dominance check works as expected. Can
> anyone
> >> help me
> >> > understand why the context contains check does not
> work the
> >> same on both
> >> > systems?
> >>
> >> That would depend entirely on how the constraint is
> written in
> >> the
> >> policy. I assume this is with the -mls policy on both?
> seinfo
> >> --constrain | grep -C1 context would show you the
> constraint
> >> in the
> >> kernel policy.
> >>
> >> Looks like refpolicy defines it as:
> >> mlsconstrain context contains
> >> (( h1 dom h2 ) and ( l1 domby l2));
> >>
> >> The 2nd part of the constraint was introduced by:
> >> commit 4c365f4a6a6f933dd13b0127e03f832c6a6cf8fc
> >> Author: Harry Ciao <qingtao.cao@xxxxxxxxxxxxx
> <mailto:qingtao.cao@xxxxxxxxxxxxx>
> >> <mailto:qingtao.cao@xxxxxxxxxxxxx
> <mailto:qingtao.cao@xxxxxxxxxxxxx>>>
> >> Date: Tue Feb 15 10:16:32 2011 +0800
> >>
> >> l1 domby l2 for contains MLS constraint
> >>
> >> As identified by Stephan Smalley, the current MLS
> >> constraint for the
> >> contains permission of the context class should
> consider
> >> the current
> >> level of a user along with the clearance level so that
> >> mls_systemlow
> >> is no longer considered contained in mls_systemhigh.
> >>
> >> Signed-off-by: Harry Ciao
> <qingtao.cao@xxxxxxxxxxxxx <mailto:qingtao.cao@xxxxxxxxxxxxx>
> >> <mailto:qingtao.cao@xxxxxxxxxxxxx
> <mailto:qingtao.cao@xxxxxxxxxxxxx>>>
> >>
> >> This was to prevent a user from logging in at a level
> below their
> >> authorized range, in the unusual scenario where the
> user's low
> >> level was
> >> not s0/systemlow.
> >>
> >> >
> >> > Ted
> >> >
> >> >
> >>
> >>
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> >>
> >> >
> >> > import selinux
> >> >
> >> > SECCLASS_CONTEXT =
> selinux.string_to_security_class("context")
> >> > CONTEXT__CONTAINS =
> >> selinux.string_to_av_perm(SECCLASS_CONTEXT, "contains")
> >> > SECCLASS_FILE = selinux.string_to_security_class("file")
> >> > FILE__READ = selinux.string_to_av_perm(SECCLASS_FILE,
> "read")
> >> >
> >> > raw_con1 = "user_u:user_r:user_t:s1"
> >> > raw_con2 = "user_u:user_r:user_t:s0"
> >> >
> >> > avd = selinux.av_decision()
> >> > selinux.avc_reset()
> >> > try:
> >> > rc = selinux.security_compute_av_raw(raw_con1,
> raw_con2,
> >> > SECCLASS_CONTEXT, CONTEXT__CONTAINS, avd)
> >> > if rc < 0:
> >> > print("selinux.security_compute_av_raw
> failed for %s
> >> %s" %
> >> > (raw_con1, raw_con2))
> >> > if (avd.allowed & CONTEXT__CONTAINS) ==
> >> CONTEXT__CONTAINS:
> >> > print("%s dominates %s" % (raw_con1, raw_con2))
> >> > else:
> >> > print("%s does not dominate %s" % (raw_con1,
> >> raw_con2))
> >> > except OSError, ex:
> >> > print "exception calling
> >> selinux.security_compute_av_raw", ex
> >> >
> >> > avd = selinux.av_decision()
> >> > selinux.avc_reset()
> >> > try:
> >> > rc = selinux.security_compute_av_raw(raw_con1,
> raw_con2,
> >> > SECCLASS_FILE, FILE__READ, avd)
> >> > if rc < 0:
> >> > print("selinux.security_compute_av_raw
> failed for %s
> >> %s" %
> >> > (raw_con1, raw_con2))
> >> > if (avd.allowed & FILE__READ) == FILE__READ:
> >> > print("%s dominates %s" % (raw_con1, raw_con2))
> >> > else:
> >> > print("%s does not dominate %s" % (raw_con1,
> >> raw_con2))
> >> >
> >> > except OSError:
> >> > print "exception calling
> >> selinux.security_compute_av_raw", ex
> >> >
> >> >
> >> >
> >> > _______________________________________________
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> >> >
> >>
> >
>
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