[PATCH 2/3] selinux: add checksum to policydb

[Sebastien Buisson]

2017-04-27 20:47 GMT+02:00 Stephen Smalley <sds at tycho.nsa.gov>:
>> I just checked, with the method of computing the checksum on a (data,
>> len) pair on entry to security_load_policy() the checksum does not
>> change after using setsebool. So it seems I would need to call
>> security_read_policy() to retrieve the binary representation of the
>> policy as currently enforced by the kernel. Unless you can see
>> another
>> way?
>
> I don't think that's a viable option, since security_read_policy() is
> going to be expensive in order to generate a full policy image, while
> security_set_bools() is supposed to be substantially cheaper than a
> full policy load.
>
> Also, the advantage of taking the hash of the original input file is
> that you can independently compute a reference hash offline or on the
> server from the same policy file and compare them and you can identify
> which policy file was loaded based on the hash.
>
> If you care about the active boolean state, then I'd suggest hashing
> the active boolean state separately and storing that after the policy
> hash.  You can do that in both security_load_policy() and
> security_set_bools().  Just iterate through the bools like
> security_set_bools() does, write the ->state of each boolean into a
> buffer, and then hash that buffer.

I just noticed another issue: with the method of computing the
checksum on a (data, len) pair on entry to security_load_policy(), the
checksum does not change after inserting a new module with semodule.
It is a problem as a module can allow actions by certain users on some
file contexts. So not detecting that kind of policy tampering defeats
the purpose of the checksum as I imagine it.

To address this I propose to come back to the idea of the notifier.
The checksum would not be stored inside the struct policydb. The
checksum would be computed on a (data, len) pair got from
security_read_policy() every time someone is asking for it through the
security_policy_cksum() hook. The ones that would potentially call
security_policy_cksum() are those that would register a callback on
lsm_notifier, and the userspace processes reading
/sys/fs/selinux/policycksum. So no matter if computing the checksum
gets expensive, that would be the caller's responsibility to use it
with care. Just like with /sys/fs/selinux/policy today in fact.

>> > You needed to get (global) enforcing mode too, didn't you?  That's
>> > separate from the policy.
>>
>> Exactly, I also need to rework the patch I proposed about this, in
>> light of the comments I received.
>
> So perhaps what you really want is a hook interface and a selinuxfs
> interface that returns a single string that encodes all of the policy
> properties that you care about?  Rather than separate hooks and
> interfaces?  You could embed the enforcing status in the string too.
> Should probably include checkreqprot as well since that affects
> enforcement of mmap/mprotect checks.

True, I should build a string of the form:
<0 or 1 for enforce>:<0 or 1 for checkreqprot>:<hashalg>=<global checksum>
I should probably rename it 'policybrief' instead of 'policycksum'.

I realize that the 'SELinux user to UNIX user' assignments are
important as well. If for instance a regular user on a given cluster
node is mapped to unconfined_u instead of user_u, this user would
erroneously have major privileges. I do not know where I should look
for this information, and possibly compute another checksum.
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