Re: MCS and default labels

[Michal Svoboda <michal.svoboda@agents.felk.cvut.cz>]

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Stephen Smalley wrote:
> The kernel MLS logic lives in security/selinux/ss/mls.c.  Determining
> the MLS level of a new subject or object is handled by
> mls_compute_sid().  Any change would have to support either model
> (inherit from source context or inherit from target context), so
> logically it would be policy-driven.  Which likely means an extension to
> the policy language and compiler.  Not an entirely trivial undertaking.

I have looked at the source code and it seems that in the said function
mls_compute_sid(), under the "case AVTAB_CHANGE:" and in the "else" path
(tclass != SECCLASS_PROCESS) the statement

mls_context_cpy_low(newcontext, scontext)

would need to be changed to

mls_context_cpy(newcontext, tcontext).

To support the original behavior, I would add a 1/0 readable/writable
file to selinuxfs, say "mls_defcontext_inherit" by mimicking the
behavior of the "enforce" file and its associated r/w functions. It
seems to me that this would suffice enough and it would avoid the need
to modify the policy language.

Would this change be acceptable?

Michal Svoboda

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