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

[Mickaël Salaün]

On Tue, Aug 06, 2024 at 11:55:27PM +0200, Jann Horn wrote:
> 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)

I was wondering if we should handle this case, but I guess it makes
sense to have a consistent policy for all kind of user-triggerable
signals.

> > > +{
> > > +       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?)
> 

I agree, let's extend landlock_file_security with a new "fown" pointer
to a Landlock domain. We'll need to call landlock_get_ruleset() in
hook_file_send_sigiotask(), and landlock_put_ruleset() in a new
hook_file_free_security().

I would be nice to to replace the redundant informations in fown_struct
but that can wait.