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?
On Tue, Sep 11, 2018 at 1:27 PM Stephen Smalley <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>> wrote:
>>
>> Understood, thanks.
>>
>> On Mon, Sep 10, 2018 at 12:46 PM Stephen Smalley <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>>
>> 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>>
>>
>> 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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