[PATCH v2] lsm: adds process attribute getter for Landlock

[Casey Schaufler]

On 5/24/2023 9:02 AM, Mickaël Salaün wrote:
>
> On 24/05/2023 17:38, Mickaël Salaün wrote:
>>
>> On 23/05/2023 23:12, Paul Moore wrote:
>>> On Tue, May 23, 2023 at 2:13 AM Jeff Xu <jeffxu at chromium.org> wrote:
>>>> On Mon, May 22, 2023 at 12:56 PM Paul Moore <paul at paul-moore.com>
>>>> wrote:
>>>>> On Thu, May 18, 2023 at 5:26 PM Casey Schaufler
>>>>> <casey at schaufler-ca.com> wrote:
>>>>>> On 5/18/2023 1:45 PM, Shervin Oloumi wrote:
>>>>>>> Adds a new getprocattr hook function to the Landlock LSM, which
>>>>>>> tracks
>>>>>>> the landlocked state of the process. This is invoked when
>>>>>>> user-space
>>>>>>> reads /proc/[pid]/attr/domain
>>>>>>
>>>>>> Please don't add a Landlock specific entry directly in the attr/
>>>>>> directory. Add it only to attr/landlock.
>>>>>>
>>>>>> Also be aware that the LSM maintainer (Paul Moore) wants to move
>>>>>> away from the /proc/.../attr interfaces in favor of a new system
>>>>>> call,
>>>>>> which is in review.
>>>>>
>>>>> What Casey said above.
>>>>>
>>>>> There is still some uncertainty around timing, and if we're perfectly
>>>>> honest, acceptance of the new syscalls at the Linus level, but yes, I
>>>>> would very much like to see the LSM infrastructure move away from
>>>>> procfs and towards a syscall API.  Part of the reasoning is that the
>>>>> current procfs API is ill-suited to handle the multiple, stacked LSMs
>>>>> and the other part being the complexity of procfs in a namespaced
>>>>> system.  If the syscall API is ultimately rejected, we will need to
>>>>> revisit the idea of a procfs API, but even then I think we'll need to
>>>>> make some changes to the current approach.
>>>>>
>>>>> As I believe we are in the latter stages of review for the syscall
>>>>> API, perhaps you could take a look and ensure that the current
>>>>> proposed API works for what you are envisioning with Landlock?
>>
>> I agree, and since the LSM syscalls are almost ready that should not
>> change much the timing. In fact, extending these syscalls might be
>> easier than tweaking the current procfs/attr API for Landlock specific
>> requirements (e.g. scoped visibility). We should ensure that these
>> syscalls would be a good fit to return file descriptors, but in the
>> short term we only need to know if a process is landlocked or not, so a
>> raw return value (0 or -errno) will be enough.
>>
>> Mentioning in the LSM syscalls patch series that they may deal with (and
>> return) file descriptors could help API reviewers though.
>
> It should be kept in mind that the current LSM syscalls only deal with
> the calling task, whereas the goal of this Landlock patch series is to
> inspect other tasks. A new LSM syscall would need to be created to
> handle pidfd e.g., named lsm_get_proc_attr() or lsm_get_pid_attr().

I think it would be lsm_get_pid_attr(). Yes, it's the obvious next step.

>
> I'm not sure if this should be a generic LSM syscall or a Landlock
> syscall though. I have plan to handle processes other than the caller
> (e.g. to restrict an existing process hierarchy), so thinking about a
> Landlock-specific syscall could make sense.
>
> To summarize, creating a new LSM syscall to deal with pidfd and to get
> LSM process "status/attr" looks OK. However, Landlock-specific
> syscalls to deal with Landlock specificities (e.g. ruleset or domain
> file descriptor) make more sense.
>
> Having one LSM-generic syscall to get minimal Landlock attributes
> (i.e. mainly to know if a process is sandboxed), and another
> Landlock-specific syscall to do more things (e.g. get the domain file
> descriptor, restrict a task) seems reasonable. The second one would
> overlap with the first one though. What do you think?

I find it difficult to think of a file descriptor as an attribute of
a process. To my (somewhat unorthodox) thinking a file descriptor is
a name for an object, not an attribute of the object. You can't access
an object by its attributes, but you can by its name. An attribute is
a description of the object. I'm perfectly happy with lsm_get_pid_attr()
returning an attribute that is a file descriptor if it describes the
process in some way, but not as a substitute for opening /proc/42.