Re: [PATCH 4/4] SELinux: allow userspace to read policy back out of the kernel

[KaiGai Kohei <kaigai@ak.jp.nec.com>]

(2010/06/16 23:53), Stephen Smalley wrote:
> On Tue, 2010-06-15 at 10:33 -0400, Eric Paris wrote:
>> On Mon, 2010-06-14 at 21:42 -0700, Casey Schaufler wrote:
>>> Eric Paris wrote:
>>>> On Mon, 2010-06-14 at 10:48 -0400, Stephen Smalley wrote:
>>>>
>>>>> On Fri, 2010-06-11 at 12:37 -0400, Eric Paris wrote:
>>>>>
>>>>>> There is interest in being able to see what the actual policy is that was
>>>>>> loaded into the kernel.  The patch creates a new selinuxfs file
>>>>>> /selinux/policy which can be read by userspace.  The actual policy that is
>>>>>> loaded into the kernel will be written back out to userspace.
>>>>>>
>>>>> Why a new node vs a read op for /selinux/load?
>>>>>
>>>>
>>>> No reason why I couldn't.  Just 'load' seemed to imply a connotation
>>>> which wasn't appropriate.  If you prefer I'll switch it when I do
>>>> another version.
>>>>
>>>
>>> If it makes any difference Smack /smack/load does read as well as write.
>>> You have the opportunity to make 2 or 3 users less confused if you do
>>> things consistently. After all, Smack uses load because SELinux does,
>>> the name is actually arbitrary, and why do something differently when
>>> it doesn't really matter?
>>
>> I did two things yesterday.  First I switch the read
>> from /selinux/policy to /selinux/load.  Then I undid that change and
>> started generating the in kernel policy buffer on open() rather than on
>> read().  It allowed me to use cat /etc/policy>  policy rather than using
>> my own half ass hacked utility.  The reason I undid the policy->load
>> change was because I didn't really want to store the old policy on open
>> if they were going to write() a new policy.  I can probably make the
>> determination based on the f_mode, but didn't really play with it yet.
>> I  try to do both in the next go-round.
> 
> Unfortunately it appears that libselinux security_load_policy() does
> open("/selinux/load", O_RDWR).  Don't ask me why.
> 
>> I'm still trying to figure out what I did to make malformed policies.
>> Must have screwed something up ripping out my prink's and debug hooks,
>> because it isn't working for me now either....
> 
> Assuming you've just reused the userspace policydb_write() code with
> minor cleanups for everything except the new ebitmap format, I'd look
> more closely there.
> KaiGai - this is the first time where we need to convert the new kernel
> ebitmap format back to the old one for generating a policy image from
> the kernel policydb that can be compared to a policy file.
> 
I have a question. Is it necessary the following command being succeeded?

  % diff /etc/selinux/targeted/policy/policy.24 /selinux/load

Although it was a few years ago I worked for the ebitmap improvement,
it seems to me the new ebitmap_write() correctly writes back entries
of the given ebitmap object.

However, it is unclear for me whether this routine can correctly repair
the original file image, or not.
For example, if startbit of each ebitmap_node was not aligned to u64
on the policy file, it will be fixed up on ebitmap_read(), so we lost
an information that what was the actual startbit of ebitmap_node in the
result.

Of course, it might be too much requirement, if we don't need the above
diff command returns success.

Thanks,
-- 
KaiGai Kohei <kaigai@ak.jp.nec.com>

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