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

[Jeff Xu]

On Thu, May 25, 2023 at 9:28 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> 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.
>
>

If I understand correctly:
1> A new lsm syscall - lsm_get_pid_attr():  Landlock will return the
process's landlock sandbox status: true/false.

Is this a right fit for SELinux to also return the process's enforcing
mode ? such as enforcing/permissive.

2> Landlock will have its own specific syscall to deal with Landlock
specificities (e.g. ruleset or domain file descriptor).