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selinux: Re: memcached permissions

Re: memcached permissions

From: KaiGai Kohei <kaigai_at_nospam>
Date: Mon Aug 16 2010 - 05:38:56 GMT
To: "Christopher J. PeBenito" <>

The attached patch is a revised version of memcached permissions.

The 'calculate' permission has gone, and INCR/DECR requires us
both of 'read' and 'write' permissions.
It means we should switch domain of the client process when we
need special treatments to unaccessable items; something like
trusted procedures.

Rest of the patch is not changed.

(2010/08/05 9:20), KaiGai Kohei wrote:
> (2010/08/04 10:25), Kelvin Edmison wrote:
>> I'm still not sure how allowing a 'calculate' permission would be helpful in
>> this case. Incr and decr allow for incrementing and decrementing by any
>> amount. There does not seem to be any real difference between that and
>> 'write' to me.
> INCR and DECR allow users to set a numerical value according to arithmetic
> rule, although SET allows to set a fully arbitrary value.
> If administrator want to allow users to modify a value in a limited way,
> he can grant 'calculate' permission, instead of 'write' permission.
> If we would be talking about RDBMS, it is possible to switch client's
> privileges during execution of a certain stored procedure.
> However, memcached does not have such a feature, so we need to consider
> more fine grained permissions.
> BTW, I noticed a different viewpoint, although I didn't reach the idea before.
> Since memcached does not have stored procedure, it might be a correct answer
> that the client switches its privileges when it tries to modify read-only
> values. Like set-uid programs in unix, SELinux has a feature to switch
> privileges of process on execve(2) time. It allows a small number of trusted
> programs to write values, but prevents to modify items by others.
>> If a strict security partitioning is desired, then perhaps a single
>> reference counter isn't feasible. Would it not be better, from a security
>> point of view, to have individual counters for the different clients?
>> The clients would have 'create|read|write' permissions, and any overall
>> administrative app could have read-only permissions on all those counters to
>> collect and sum (or otherwise report) them?
> If a strict security partitioning environment, it seems to me what you introduced
> is reasonable.
> Thanks,
>> Kelvin
>> On 02/08/10 1:45 AM, "KaiGai Kohei"<> wrote:
>>> (2010/07/30 22:55), Kelvin Edmison wrote:
>>>> While I haven't yet read the patch, I would like to understand why there is
>>>> a need for a Calculate permission. Why would someone be granted 'calculate'
>>>> permission but not 'write' permission?
>>>> Kelvin
>>> The issue depends on individual user's requirement of security.
>>> If they want not to leak anything over the security domains,
>>> they should grant the 'calculate' permission on everybody who
>>> already have both 'read' and 'write' permissions.
>>> It it equivalent to these permissions.
>>> However, it may lack flexibility in configuration of access
>>> controls, if users don't consider 'INCR' and 'DECR' are risk
>>> information leaks/manipulations.
>>> For example, it is not a rare case that we don't want to expose
>>> individual client's items, but want to control a shared reference
>>> counter.
>>> Ideally, I'd like to define more fine grained permissions to
>>> distinguish a security sensitive operations and others.
>>> But here is limitation of protocol. We cannot automatically
>>> determine what is security sensitive data and what is not.
>>> Thanks,
>>>> On 30/07/10 12:49 AM, "KaiGai Kohei"<> wrote:
>>>>> I'll mainly submit the patch and message to SELinux community,
>>>>> but please don't hesitate to comment anything from memcached
>>>>> community.
>>>>> --------
>>>>> The attached patch adds policies to support access controls
>>>>> on key-value items managed by memcached with SELinux engine.
>>>>> Nowadays, various kind of key-value-stores support memcached
>>>>> compatible protocol as a de-facto standard. So, it will be a
>>>>> reasonable start to consider the protocol to control accesses
>>>>> from clients; typically web applications.
>>>>> 1) new permissions
>>>>> This patch adds 'kv_item' class with the following permissions
>>>>> - create
>>>>> - getattr
>>>>> - setattr
>>>>> - remove
>>>>> - relabelfrom
>>>>> - relabelto
>>>>> - read
>>>>> - write
>>>>> - append
>>>>> - calculate
>>>>> Most of permission works as literal.
>>>>> On the 'SET' or 'CAS' operations, it creates a new item when here
>>>>> is no items with same kye. In this case, 'create' permission shall
>>>>> be checked. Elsewhere, 'write' permission shall be checked on the
>>>>> existing items.
>>>>> When an item get expired, it shall be unlinked internally. In this
>>>>> case, no permissions are checked, because it does not work according
>>>>> to the user's request.
>>>>> On the 'FLUSH_ALL' operation, it unlinks any items older than
>>>>> a certain watermark. In this case, 'remove' permission shall be
>>>>> checked on the items to be unlinked. If violated, it skips to
>>>>> unlink this item.
>>>>> On 'INCR' or 'DECR' operation, 'calculate' permission shall be checked.
>>>>> Is it necessary to distinguish between 'INCR' and 'DECR' here?
>>>>> E.g, an item which can be incremented, but unavailable to decrement.
>>>>> 2) new types
>>>>> - memcached_db_t
>>>>> Some of modular memcached engines support on-disk storage, not only
>>>>> volatile ram. The allows us to use a certain file
>>>>> as a backend storage, but existing policy does not have definition
>>>>> of data file type. This type allows memcached_t read/write accesses.
>>>>> - memcached_item_t (default of unconfined domain)
>>>>> - memcached_ro_item_t
>>>>> - memcached_secret_item_t
>>>>> - user_memcached_item_t (default of rbac domain)
>>>>> - unpriv_memcached_item_t (default of unprivileged domain)
>>>>> These are types of individual key-value items.
>>>>> The three default types are read-writable for its domains,
>>>>> memcached_ro_item_t is read-only for confined domains, and
>>>>> memcached_secret_t is invisible from confined domains.
>>>>> 3) supplemental policies
>>>>> - This patch also adds permission on memcached_t to communicate with
>>>>> SELinux using netlink socket and selinuxfs.
>>>>> - This patch also adds permission on memcached_t to write out audit
>>>>> logs onto auditd daemon, although it is not implemented yet.
>>>>> Thanks, Any comments please.

-- KaiGai Kohei <>

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