SELinux: How to split permissions for keys?

Mon Feb 3 13:13:13 UTC 2020

On 2/2/20 2:30 PM, Richard Haines wrote:
> On Thu, 2020-01-23 at 15:35 -0500, Stephen Smalley wrote:
>> On 1/23/20 10:46 AM, Stephen Smalley wrote:
>>> On 1/23/20 10:12 AM, David Howells wrote:
>>>> Hi Stephen,
>>>>
>>>> I have patches to split the permissions that are used for keys to
>>>> make
>>>> them a
>>>> bit finer grained and easier to use - and also to move to ACLs
>>>> rather
>>>> than
>>>> fixed masks.  See patch "keys: Replace uid/gid/perm permissions
>>>> checking with
>>>> an ACL" here:
>>>>
>>>>      
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=keys-acl
>>>>   
>>>>
>>>>
>>>> However, I may not have managed the permission mask
>>>> transformation inside
>>>> SELinux correctly.  Could you lend an eyeball?  The change to
>>>> the
>>>> permissions
>>>> model is as follows:
>>>>
>>>>       The SETATTR permission is split to create two new
>>>> permissions:
>>>>        (1) SET_SECURITY - which allows the key's owner, group and
>>>> ACL
>>>> to be
>>>>            changed and a restriction to be placed on a keyring.
>>>>        (2) REVOKE - which allows a key to be revoked.
>>>>       The SEARCH permission is split to create:
>>>>        (1) SEARCH - which allows a keyring to be search and a key
>>>> to be
>>>> found.
>>>>        (2) JOIN - which allows a keyring to be joined as a
>>>> session
>>>> keyring.
>>>>        (3) INVAL - which allows a key to be invalidated.
>>>>       The WRITE permission is also split to create:
>>>>        (1) WRITE - which allows a key's content to be altered and
>>>> links
>>>> to be
>>>>            added, removed and replaced in a keyring.
>>>>        (2) CLEAR - which allows a keyring to be cleared
>>>> completely.
>>>> This is
>>>>            split out to make it possible to give just this to an
>>>> administrator.
>>>>        (3) REVOKE - see above.
>>>>
>>>> The change to SELinux is attached below.
>>>>
>>>> Should the split be pushed down into the SELinux policy rather
>>>> than
>>>> trying to
>>>> calculate it?
>>>
>>> My understanding is that you must provide full backward
>>> compatibility
>>> with existing policies; hence, you must ensure that you always
>>> check the
>>> same SELinux permission(s) for the same operation when using an
>>> existing
>>> policy.
>>>
>>> In order to support finer-grained distinctions in SELinux with
>>> future
>>> policies, you can define a new SELinux policy capability along with
>>> the
>>> new permissions, and if the policy capability is enabled in the
>>> policy,
>>> check the new permissions rather than the old ones. A recent
>>> example of
>>> adding a new policy capability and using it can be seen in:
>>> https://lore.kernel.org/selinux/20200116194530.8696-1-jeffv@google.com/T/#u
>>> although that patch was rejected for other reasons.
>>>
>>> Another example was when we introduced fine-grained distinctions
>>> for all
>>> network address families, commit
>>> da69a5306ab92e07224da54aafee8b1dccf024f6.
>>>
>>> The new policy capability also needs to be defined in libsepol for
>>> use
>>> by the policy compiler; an example can be seen in:
>>> https://lore.kernel.org/selinux/20170714164801.6346-1-sds@tycho.nsa.gov/
>>>
>>> Then future policies can declare the policy capability when they
>>> are
>>> ready to start using the new permissions instead of the old.
>>>
>>>> Thanks,
>>>> David
>>>> ---
>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>>> index 116b4d644f68..c8db5235b01f 100644
>>>> --- a/security/selinux/hooks.c
>>>> +++ b/security/selinux/hooks.c
>>>> @@ -6556,6 +6556,7 @@ static int
>>>> selinux_key_permission(key_ref_t
>>>> key_ref,
>>>>    {
>>>>        struct key *key;
>>>>        struct key_security_struct *ksec;
>>>> +    unsigned oldstyle_perm;
>>>>        u32 sid;
>>>>        /* if no specific permissions are requested, we skip the
>>>> @@ -6564,13 +6565,26 @@ static int
>>>> selinux_key_permission(key_ref_t
>>>> key_ref,
>>>>        if (perm == 0)
>>>>            return 0;
>>>> +    oldstyle_perm = perm & (KEY_NEED_VIEW | KEY_NEED_READ |
>>>> KEY_NEED_WRITE |
>>>> +                KEY_NEED_SEARCH | KEY_NEED_LINK);
>>>> +    if (perm & KEY_NEED_SETSEC)
>>>> +        oldstyle_perm |= OLD_KEY_NEED_SETATTR;
>>>> +    if (perm & KEY_NEED_INVAL)
>>>> +        oldstyle_perm |= KEY_NEED_SEARCH;
>>>> +    if (perm & KEY_NEED_REVOKE && !(perm &
>>>> OLD_KEY_NEED_SETATTR))
>>>> +        oldstyle_perm |= KEY_NEED_WRITE;
>>>> +    if (perm & KEY_NEED_JOIN)
>>>> +        oldstyle_perm |= KEY_NEED_SEARCH;
>>>> +    if (perm & KEY_NEED_CLEAR)
>>>> +        oldstyle_perm |= KEY_NEED_WRITE;
>>>> +
>>>
>>> I don't know offhand if this ensures that the same SELinux
>>> permission is
>>> always checked as it would have been previously for the same
>>> operation+arguments.  That's what you have to preserve for
>>> existing
>>> policies.
>>
>> As Richard pointed out in his email, your key-acl series replaces
>> two
>> different old permissions (LINK, SEARCH) with a single permission
>> (JOIN)
>> in different callers, so by the time we reach the SELinux hook we
>> cannot
>> map it back unambiguously and provide full backward
>> compatibility.  The
>> REVOKE case also seems fragile although there you seem to distinguish
>> by
>> sometimes passing in OLD_KEY_NEED_SETATTR and sometimes not?  You'll
>> have to fix the JOIN case to avoid userspace breakage.
>>
>> You may want to go ahead and explicitly translate all of the
>> KEY_NEED
>> permissions to SELinux permissions rather than passing the key
>> permissions directly here to avoid requiring that the values always
>> match.  The SELinux permission symbols are of the form
>> CLASS__PERMISSION
>> (NB double underscore), e.g. KEY__SETATTR, generated automatically
>> from
>> the security/selinux/include/classmap.h tables to the
>> security/selinux/av_permissions.h generated header. Most hooks
>> perform
>> such translation, e.g. file_mask_to_av().  You will almost certainly
>> need to do this if/when you introduce support for the new permissions
>> to
>> SELinux.
> 
> 
> This problem has now been fixed in [1].
> It passes the current selinux-test-suite (except test/fs_filesystem
> regression).
> 
> As the fix now includes a new 'key_perms' policy capability to allow
> use of the extended key permissions, I've updated libsepol and the
> selinux-testsuite test/keys to test these.
> 
> I'll submit two RFC patches that will allow [1] to be tested with
> 'key_perms' true or false.
> 
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=keys-next

Was that kernel patch ever posted to selinux list and/or the selinux 
kernel maintainers?  I don't recall seeing it.  If not, please send it 
to the selinux list for review; at least one selinux maintainer should 
ack it before it gets accepted into any other tree.