Re: [PATCH v13 3/4] selinux: teach SELinux about anonymous inodes

[Stephen Smalley <stephen.smalley.work@gmail.com>]

On Fri, Jan 8, 2021 at 3:17 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> On Fri, Jan 8, 2021 at 11:35 AM Stephen Smalley
> <stephen.smalley.work@gmail.com> wrote:
> >
> > On Wed, Jan 6, 2021 at 10:03 PM Paul Moore <paul@paul-moore.com> wrote:
> > >
> > > On Wed, Nov 11, 2020 at 8:54 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
> > > > From: Daniel Colascione <dancol@google.com>
> > > >
> > > > This change uses the anon_inodes and LSM infrastructure introduced in
> > > > the previous patches to give SELinux the ability to control
> > > > anonymous-inode files that are created using the new
> > > > anon_inode_getfd_secure() function.
> > > >
> > > > A SELinux policy author detects and controls these anonymous inodes by
> > > > adding a name-based type_transition rule that assigns a new security
> > > > type to anonymous-inode files created in some domain. The name used
> > > > for the name-based transition is the name associated with the
> > > > anonymous inode for file listings --- e.g., "[userfaultfd]" or
> > > > "[perf_event]".
> > > >
> > > > Example:
> > > >
> > > > type uffd_t;
> > > > type_transition sysadm_t sysadm_t : anon_inode uffd_t "[userfaultfd]";
> > > > allow sysadm_t uffd_t:anon_inode { create };
> > > >
> > > > (The next patch in this series is necessary for making userfaultfd
> > > > support this new interface.  The example above is just
> > > > for exposition.)
> > > >
> > > > Signed-off-by: Daniel Colascione <dancol@google.com>
> > > > Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> > > > ---
> > > >  security/selinux/hooks.c            | 56 +++++++++++++++++++++++++++++
> > > >  security/selinux/include/classmap.h |  2 ++
> > > >  2 files changed, 58 insertions(+)
> > > >
> > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > > index 6b1826fc3658..d092aa512868 100644
> > > > --- a/security/selinux/hooks.c
> > > > +++ b/security/selinux/hooks.c
> > > > @@ -2927,6 +2927,61 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
> > > >         return 0;
> > > >  }
> > > >
> > > > +static int selinux_inode_init_security_anon(struct inode *inode,
> > > > +                                           const struct qstr *name,
> > > > +                                           const struct inode *context_inode)
> > > > +{
> > > > +       const struct task_security_struct *tsec = selinux_cred(current_cred());
> > > > +       struct common_audit_data ad;
> > > > +       struct inode_security_struct *isec;
> > > > +       int rc;
> > > > +
> > > > +       if (unlikely(!selinux_initialized(&selinux_state)))
> > > > +               return 0;
> > > > +
> > > > +       isec = selinux_inode(inode);
> > > > +
> > > > +       /*
> > > > +        * We only get here once per ephemeral inode.  The inode has
> > > > +        * been initialized via inode_alloc_security but is otherwise
> > > > +        * untouched.
> > > > +        */
> > > > +
> > > > +       if (context_inode) {
> > > > +               struct inode_security_struct *context_isec =
> > > > +                       selinux_inode(context_inode);
> > > > +               if (context_isec->initialized != LABEL_INITIALIZED)
> > > > +                       return -EACCES;
> Stephen, as per your explanation below, is this check also
> problematic? I mean is it possible that /dev/kvm context_inode may not
> have its label initialized? If so, then v12 of the patch series can be
> used as is. Otherwise, I will send the next version which rollbacks
> v14 and v13, except for this check. Kindly confirm.

The context_inode should always be initialized already.  I'm not fond
though of silently returning -EACCES here.  At the least we should
have a pr_err() or pr_warn() here.  In reality, this could only occur
in the case of a kernel bug or memory corruption so it used to be a
candidate for WARN_ON() or BUG_ON() or similar but I know that
BUG_ON() at least is frowned upon these days.