[RFC PATCH 2/4] selinux: clarify task subjective and objective credentials

[Paul Moore]

On Tue, Mar 9, 2021 at 10:06 PM John Johansen
<john.johansen at canonical.com> wrote:
> On 2/19/21 3:29 PM, Paul Moore wrote:
> > SELinux has a function, task_sid(), which returns the task's
> > objective credentials, but unfortunately is used in a few places
> > where the subjective task credentials should be used.  Most notably
> > in the new security_task_getsecid_subj() LSM hook.
> >
> > This patch fixes this and attempts to make things more obvious by
> > introducing a new function, task_sid_subj(), and renaming the
> > existing task_sid() function to task_sid_obj().
> >
> > Signed-off-by: Paul Moore <paul at paul-moore.com>
>
> I have a couple of questions below but the rest looks good
>
> > ---
> >  security/selinux/hooks.c |   85 +++++++++++++++++++++++++++-------------------
> >  1 file changed, 49 insertions(+), 36 deletions(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index f311541c4972e..1c53000d28e37 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -229,10 +229,23 @@ static inline u32 cred_sid(const struct cred *cred)
> >       return tsec->sid;
> >  }
> >
> > +/*
> > + * get the subjective security ID of a task
> > + */
> > +static inline u32 task_sid_subj(const struct task_struct *task)
> > +{
> > +     u32 sid;
> > +
> > +     rcu_read_lock();
> > +     sid = cred_sid(rcu_dereference(task->cred));
> > +     rcu_read_unlock();
> > +     return sid;
> > +}
> > +
> >  /*
> >   * get the objective security ID of a task
> >   */
> > -static inline u32 task_sid(const struct task_struct *task)
> > +static inline u32 task_sid_obj(const struct task_struct *task)
> >  {
> >       u32 sid;
> >
> > @@ -2034,11 +2047,8 @@ static inline u32 open_file_to_av(struct file *file)
> >
> >  static int selinux_binder_set_context_mgr(struct task_struct *mgr)
> >  {
> > -     u32 mysid = current_sid();
> > -     u32 mgrsid = task_sid(mgr);
> > -
> >       return avc_has_perm(&selinux_state,
> > -                         mysid, mgrsid, SECCLASS_BINDER,
> > +                         current_sid(), task_sid_obj(mgr), SECCLASS_BINDER,
> >                           BINDER__SET_CONTEXT_MGR, NULL);
> >  }
> >
> > @@ -2046,8 +2056,8 @@ static int selinux_binder_transaction(struct task_struct *from,
> >                                     struct task_struct *to)
> >  {
> >       u32 mysid = current_sid();
> > -     u32 fromsid = task_sid(from);
> > -     u32 tosid = task_sid(to);
> > +     u32 fromsid = task_sid_subj(from);
>
> fromsid potentially gets used as both the subject and the object the following
> permission checks. It makes sense to use the same cred for both checks but
> what I am not sure about yet is whether its actually safe to use the subject
> sid when the task isn't current.
>
> ie. I am still trying to determine if there is a race here between the transaction
> request and the permission check.

Okay, I see what you are concerned about now ... and unfortunately I'm
not seeing a lot of precedence in the kernel for this type of usage
either; the closest I can find is something like task_lock(), but that
doesn't seem to cover the subjective creds.  In fact, looking at
override_creds(), there is nothing preventing a task from changing
it's subjective creds at any point in time.

Beyond the task_sid_subj() code here, looking back at patch 1 and the
use of security_task_getsecid_subj() we look to be mostly safe (where
safe means we are only inspecting the current task) with the exception
of the binder code once again.  There are some other exceptions but
they are in the ptrace and audit code, both of which should be okay
given the nature and calling context of the code.

The problem really does seem to be just binder, and as I look at
binder userspace example code, I'm starting to wonder if binder is
setup properly to operate sanely in a situation where a process
overrides its subject creds.  It may be that we always need to use the
objective/real creds with binder.  Jeff, any binder insight here you
can share with us?

> > +     u32 tosid = task_sid_subj(to);
> its not clear to me that using the subj for to is correct

Yes, I believe you are correct.  Jeff, I know you looked at this code
already, but I'm guessing you may have missed this (just as I did when
I wrote it); are you okay with changing 'tosid' in
selinux_binder_transaction() to the task's objective credentials?

--
paul moore
www.paul-moore.com