Re: [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver #2]

[Stephen Smalley <sds@tycho.nsa.gov>]

On 6/5/19 3:28 PM, Greg KH wrote:
> On Wed, Jun 05, 2019 at 02:25:33PM -0400, Stephen Smalley wrote:
>> On 6/5/19 1:47 PM, Andy Lutomirski wrote:
>>>
>>>> On Jun 5, 2019, at 10:01 AM, Casey Schaufler <casey@schaufler-ca.com> wrote:
>>>>
>>>>> On 6/5/2019 9:04 AM, Andy Lutomirski wrote:
>>>>>> On Wed, Jun 5, 2019 at 7:51 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
>>>>>>> On 6/5/2019 1:41 AM, David Howells wrote:
>>>>>>> Casey Schaufler <casey@schaufler-ca.com> wrote:
>>>>>>>
>>>>>>>> I will try to explain the problem once again. If process A
>>>>>>>> sends a signal (writes information) to process B the kernel
>>>>>>>> checks that either process A has the same UID as process B
>>>>>>>> or that process A has privilege to override that policy.
>>>>>>>> Process B is passive in this access control decision, while
>>>>>>>> process A is active. In the event delivery case, process A
>>>>>>>> does something (e.g. modifies a keyring) that generates an
>>>>>>>> event, which is then sent to process B's event buffer.
>>>>>>> I think this might be the core sticking point here.  It looks like two
>>>>>>> different situations:
>>>>>>>
>>>>>>> (1) A explicitly sends event to B (eg. signalling, sendmsg, etc.)
>>>>>>>
>>>>>>> (2) A implicitly and unknowingly sends event to B as a side effect of some
>>>>>>>       other action (eg. B has a watch for the event A did).
>>>>>>>
>>>>>>> The LSM treats them as the same: that is B must have MAC authorisation to send
>>>>>>> a message to A.
>>>>>> YES!
>>>>>>
>>>>>> Threat is about what you can do, not what you intend to do.
>>>>>>
>>>>>> And it would be really great if you put some thought into what
>>>>>> a rational model would be for UID based controls, too.
>>>>>>
>>>>>>> But there are problems with not sending the event:
>>>>>>>
>>>>>>> (1) B's internal state is then corrupt (or, at least, unknowingly invalid).
>>>>>> Then B is a badly written program.
>>>>> Either I'm misunderstanding you or I strongly disagree.
>>>>
>>>> A program needs to be aware of the conditions under
>>>> which it gets event, *including the possibility that
>>>> it may not get an event that it's not allowed*. Do you
>>>> regularly write programs that go into corrupt states
>>>> if an open() fails? Or where read() returns less than
>>>> the amount of data you ask for?
>>>
>>> I do not regularly write programs that handle read() omitting data in the middle of a TCP stream.  I also don’t write programs that wait for processes to die and need to handle the case where a child is dead, waitid() can see it, but SIGCHLD wasn’t sent because “security”.
>>>
>>>>
>>>>>    If B has
>>>>> authority to detect a certain action, and A has authority to perform
>>>>> that action, then refusing to notify B because B is somehow missing
>>>>> some special authorization to be notified by A is nuts.
>>>>
>>>> You are hand-waving the notion of authority. You are assuming
>>>> that if A can read X and B can read X that A can write B.
>>>
>>> No, read it again please. I’m assuming that if A can *write* X and B can read X then A can send information to B.
>>
>> I guess the questions here are:
>>
>> 1) How do we handle recursive notification support, since we can't check
>> that B can read everything below a given directory easily?  Perhaps we can
>> argue that if I have watch permission to / then that implies visibility to
>> everything below it but that is rather broad.
> 
> How do you handle fanotify today which I think can do this?

Doesn't appear to have been given much thought; looks like 
fanotify_init() checks capable(CAP_SYS_ADMIN) and fanotify_mark() checks 
inode_permission(MAY_READ) on the mount/directory/file.  File 
descriptors for monitored files returned upon events at least get vetted 
through security_file_open() so that can prevent the monitoring process 
from receiving arbitrary descriptors. Would be preferable if 
fanotify_mark() did some kind of security_path_watch() or similar check, 
and distinguished mounts versus directories since monitoring of 
directories is not recursive.