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

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

On Fri, Jan 8, 2021 at 2:35 PM 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;
> > > +
> > > +               isec->sclass = context_isec->sclass;
> >
> > Taking the object class directly from the context_inode is
> > interesting, and I suspect problematic.  In the case below where no
> > context_inode is supplied the object class is set to
> > SECCLASS_ANON_INODE, which is correct, but when a context_inode is
> > supplied there is no guarantee that the object class will be set to
> > SECCLASS_ANON_INODE.  This could both pose a problem for policy
> > writers (how do you distinguish the anon inode from other normal file
> > inodes in this case?) as well as an outright fault later in this
> > function when we try to check the ANON_INODE__CREATE on an object
> > other than a SECCLASS_ANON_INODE object.
> >
> > It works in the userfaultfd case because the context_inode is
> > originally created with this function so the object class is correctly
> > set to SECCLASS_ANON_INODE, but can we always guarantee that to be the
> > case?  Do we ever need or want to support using a context_inode that
> > is not SECCLASS_ANON_INODE?
>
> Sorry, I haven't been following this.  IIRC, the original reason for
> passing a context_inode was to support the /dev/kvm or similar use
> cases where the driver is creating anonymous inodes to represent
> specific objects/interfaces derived from the device node and we want
> to be able to control subsequent ioctl operations on those anonymous
> inodes in the same manner as for the device node.  For example, ioctl
> operations on /dev/kvm can end up returning file descriptors for
> anonymous inodes representing a specific VM or VCPU or similar.  If we
> propagate the security class and SID from the /dev/kvm inode (the
> context inode) to the new anonymous inode, we can write a single
> policy rule over all ioctl operations related to /dev/kvm.

Thanks for the background, and the /dev/kvm example, that is what I was missing.

> That's
> also why we used the FILE__CREATE permission here originally; that was
> also intentional.  All the file-related classes including anon_inode
> inherit a common set of file permissions including create and thus we
> often use the FILE__<permission> in common code when checking
> permission against any potentially derived class.

Yes, if all of the anonymous inodes are not going to fall into the
anon_inode object class then FILE__CREATE makes the most sense.

Thanks Stephen.

-- 
paul moore
www.paul-moore.com