Re: Problem with 9ba09998baa9 ("selinux: Implement the watch_key security hook") in linux-next

[Paul Moore <paul@paul-moore.com>]

On Wed, Apr 22, 2020 at 3:20 PM Paul Moore <paul@paul-moore.com> wrote:
>
> On Tue, Apr 21, 2020 at 8:29 AM David Howells <dhowells@redhat.com> wrote:
> > Paul Moore <paul@paul-moore.com> wrote:
> >
> > > ... in particular it is the fifth argument to avc_has_perm(),
> > > "KEY_NEED_VIEW" which is a problem.  KEY_NEED_VIEW is not a SELinux
> > > permission and would likely result in odd behavior when passed to
> > > avc_has_perm().
> >
> > I think it works because KEY_NEED_VIEW happens to coincide with KEY__VIEW.  It
> > should just use KEY__VIEW instead.
>
> Yes, it looks like it.  To be clear, it is dangerous to rely on
> permission values from outside SELinux aligning with SELinux
> permissions; changing the outside permission values w/o adjusting the
> SELinux hook code to do the necessary translation will result in some
> scary behavior (wrong permission checks).
>
> > > it probably makes the most sense to pull the permission mapping in
> > > selinux_key_permission() out into a separate function, e.g.
> > > key_perm_to_av(...)
> >
> > Agreed.  How about the attached patch?  I wonder if I should do bit-by-bit
> > translation rather than using a switch?  But then it's difficult to decide
> > what it means if someone passes in two KEY_NEED_* flags OR'd together - is it
> > either or both?
>
> Comments inline.
>
> > > and then use this newly created mapping function in [...]
> > > selinux_watch_key()
> >
> > No, I think I should just hard-code KEY__VIEW there.
>
> FWIW, my comment was based on a version of linux-next where you were
> making policycap based permission adjustments to KEY_VIEW and I
> thought you would want the same adjustments to be applied to both
> access control points.  That code appears to now be gone in
> linux-next.
>
> > ---
> > commit 70d1d82aa014ae4511976b4d80c17138006dddec
> > Author: David Howells <dhowells@redhat.com>
> > Date:   Tue Apr 21 13:15:16 2020 +0100
> >
> >     selinux: Fix use of KEY_NEED_* instead of KEY__* perms
> >
> >     selinux_key_getsecurity() is passing the KEY_NEED_* permissions to
> >     security_sid_to_context() instead of the KEY__* values.  It happens to work
> >     because the values are all coincident.
> >
> >     Fixes: d720024e94de ("[PATCH] selinux: add hooks for key subsystem")
> >     Reported-by: Paul Moore <paul@paul-moore.com>
> >     Signed-off-by: David Howells <dhowells@redhat.com>
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 0b4e32161b77..32f7fa538c5f 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -6539,12 +6539,27 @@ static void selinux_key_free(struct key *k)
> >         kfree(ksec);
> >  }
> >
> > +static unsigned int selinux_keyperm_to_av(unsigned int need_perm)
> > +{
> > +       switch (need_perm) {
> > +       case KEY_NEED_VIEW:     return KEY__VIEW;
> > +       case KEY_NEED_READ:     return KEY__READ;
> > +       case KEY_NEED_WRITE:    return KEY__WRITE;
> > +       case KEY_NEED_SEARCH:   return KEY__SEARCH;
> > +       case KEY_NEED_LINK:     return KEY__LINK;
> > +       case KEY_NEED_SETATTR:  return KEY__SETATTR;
> > +       default:
> > +               return 0;
> > +       }
>
> Regarding your question of permission translation via switch-statement
> as opposed to bit-by-bit comparison, I think it depends on if multiple
> permissions are going to be required in a single call to the hook.
> The failure mode for the code above if multiple permissions are
> requested is not very good, it defaults to *no* permission which means
> that if someone requested KEY_NEED_SEARCH|KEY_NEED_VIEW (or some other
> combination) then the SELinux check would not check any permissions
> ... that seems wrong to me.
>
> If we want to stick with a switch statement I think we should have it
> return -EPERM for the default case to protect against this.  We don't
> need the full 32-bits afforded us by the unsigned int.
>
> > +}
> > +
> >  static int selinux_key_permission(key_ref_t key_ref,
> >                                   const struct cred *cred,
> > -                                 unsigned perm)
> > +                                 unsigned need_perm)
> >  {
> >         struct key *key;
> >         struct key_security_struct *ksec;
> > +       unsigned int perm;
> >         u32 sid;
> >
> >         /* if no specific permissions are requested, we skip the
> > @@ -6553,6 +6568,7 @@ static int selinux_key_permission(key_ref_t key_ref,
> >         if (perm == 0)
> >                 return 0;
> >
> > +       perm = selinux_keyperm_to_av(need_perm);
>
> ... and add a check for (perm < 0) as discussed above if we stick with
> the switch statement.

... and we should probably emit some sort of message to indicate that
an invalid permission set was used.

-- 
paul moore
www.paul-moore.com