Re: [PATCH v6 1/2] selinux: add brief info to policydb

[Sebastien Buisson]

2017-05-23 21:53 GMT+02:00 Stephen Smalley <sds@xxxxxxxxxxxxx>:
> On Tue, 2017-05-23 at 18:29 +0200, Sebastien Buisson wrote:
>> Hi,
>>
>> 2017-05-18 23:49 GMT+02:00 Paul Moore <paul@xxxxxxxxxxxxxx>:
>> > My apologies to you and Sebastien for not reviewing these patches
>> > sooner.
>>
>> It is ok, no problem.
>> Thanks for all the advice from you and Stephen. I will try to take
>> all
>> this into account.
>>
>> As I understand it, I should not give the choice to allocate or not
>> the string returned by security_policydb_brief(). The initial reason
>> for this was that Lustre client code is expected to retrieve policy
>> brief info hundreds or thousands of times per second, so saving on
>> memory allocation would make sense. So if security_policydb_brief()
>> necessarily allocates memory for the string returned, and I
>> appreciate
>> it helps maintenance and avoids complexity, it should not be called
>> so
>> often.
>> One way to tackle this is to rely on the notification system: Lustre
>> client code would call security_policydb_brief() only when it gets a
>> change notification, and stores the current policy brief info
>> internally.
>> Another way could be to add another hook to check policy brief info
>> validity. It would take a string as an input parameter, and return 0
>> if it matches the current policy. So Lustre client code would
>> systematically call this hook, and only call
>> security_policydb_brief()
>> when the policy has changed, to store the current value internally.
>>
>> I have recently identified a new need from Lustre client code. We
>> need
>> to protect against the case where the policy is changed or set in
>> permissive mode, and then set back to its previous state, to
>> workaround policy check as carried out on server side based on policy
>> brief info sent by client. In this scenario, the policy would only be
>> the expected one by the time the client sends a request to the server
>> (for instance a file open request), but not after that when SELinux
>> actually checks the permissions on the client (via
>> security_file_open() in this example).
>> A solution to address this could be to add a new parameter to
>> security_policydb_brief() hook, in the form of a pointer to an
>> integer
>> giving the current sequence number of the policy. That would
>> complement the policy brief info, with the notion of change to the
>> policy. I do not think it is desirable to include the sequence number
>> in the policy brief info, as it is not the essence of the policy.
>> Now with this sequence info in mind, the new hook to check policy
>> brief info validity would only need to check the sequence, instead of
>> the policy brief string. The current value of the sequence info
>> should
>> be stored by Lustre internally, and checked after SELinux permission
>> checks. If a change is detected, Lustre client must stop normal
>> processing and return an error for the current request.
>
> Not sure about your threat model but I think you are fighting a losing
> battle there.  A malicious admin has too many ways to defeat your
> checking.

I do not have much experience in the domain, so every advice or idea
is appreciated.
If what I want to accomplish is implemented using the notification
system, what would be the possibility for a malicious admin to change
the policy or the way it is enforced without triggering a
notification? Of course, the places in the SELinux kernel code from
where to trigger the notification have to be thought thoroughly.

> Relying on the seqno also seems brittle; you could easily end up
> causing a client to fail just because a policy update happened to be
> installed at the same time, even though there was nothing wrong or
> malicious about the policy update itself.

I consider it is not a problem if one request must be resent, even if
the policy update was legitimate. It would not disturb Lustre
behavior, and it might be preferable to proceed to a resend instead of
missing a potential fraud.

Thanks,
Sebastien.