[PATCH v2 1/4] Landlock: Add signal control

[Jann Horn]

On Tue, Aug 6, 2024 at 8:56 PM Jann Horn <jannh at google.com> wrote:
> On Tue, Aug 6, 2024 at 8:11 PM Tahera Fahimi <fahimitahera at gmail.com> wrote:
> > Currently, a sandbox process is not restricted to send a signal
> > (e.g. SIGKILL) to a process outside of the sandbox environment.
> > Ability to sending a signal for a sandboxed process should be
> > scoped the same way abstract unix sockets are scoped. Therefore,
> > we extend "scoped" field in a ruleset with
> > "LANDLOCK_SCOPED_SIGNAL" to specify that a ruleset will deny
> > sending any signal from within a sandbox process to its
> > parent(i.e. any parent sandbox or non-sandboxed procsses).
[...]
> > +       if (is_scoped)
> > +               return 0;
> > +
> > +       return -EPERM;
> > +}
> > +
> > +static int hook_file_send_sigiotask(struct task_struct *tsk,
> > +                                   struct fown_struct *fown, int signum)
> > +{
> > +       bool is_scoped;
> > +       const struct landlock_ruleset *dom, *target_dom;
> > +       struct task_struct *result = get_pid_task(fown->pid, fown->pid_type);
>
> I'm not an expert on how the fowner stuff works, but I think this will
> probably give you "result = NULL" if the file owner PID has already
> exited, and then the following landlock_get_task_domain() would
> probably crash? But I'm not entirely sure about how this works.
>
> I think the intended way to use this hook would be to instead use the
> "file_set_fowner" hook to record the owning domain (though the setup
> for that is going to be kind of a pain...), see the Smack and SELinux
> definitions of that hook. Or alternatively maybe it would be even
> nicer to change the fown_struct to record a cred* instead of a uid and
> euid and then use the domain from those credentials for this hook...
> I'm not sure which of those would be easier.

(For what it's worth, I think the first option would probably be
easier to implement and ship for now, since you can basically copy
what Smack and SELinux are already doing in their implementations of
these hooks. I think the second option would theoretically result in
nicer code, but it might require a bit more work, and you'd have to
include the maintainers of the file locking code in the review of such
refactoring and have them approve those changes. So if you want to get
this patchset into the kernel quickly, the first option might be
better for now?)