On Wed, Jan 6, 2021 at 7:03 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 11, 2020 at 8:54 PM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
> > From: Daniel Colascione <dancol@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> > ---
> > 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.
>
Thanks for catching this. I'll initialize 'sclass' unconditionally to
SECCLASS_ANON_INODE in the next version. Also, do you think I should
add a check that context_inode's sclass must be SECCLASS_ANON_INODE to
confirm that we never receive a regular inode as context_inode?
> 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?
>
I don't think there is any requirement of supporting context_inode
which isn't anon-inode. And even if there is, as you described
earlier, for ANON_INODE__CREATE to work the sclass has to be
SECCLASS_ANON_INODE. I'll appreciate comments on this from others,
particularly Daniel and Stephen who originally discussed and
implemented this patch.
> > + isec->sid = context_isec->sid;
> > + } else {
> > + isec->sclass = SECCLASS_ANON_INODE;
> > + rc = security_transition_sid(
> > + &selinux_state, tsec->sid, tsec->sid,
> > + isec->sclass, name, &isec->sid);
> > + if (rc)
> > + return rc;
> > + }
> > +
> > + isec->initialized = LABEL_INITIALIZED;
> > +
> > + /*
> > + * Now that we've initialized security, check whether we're
> > + * allowed to actually create this type of anonymous inode.
> > + */
> > +
> > + ad.type = LSM_AUDIT_DATA_INODE;
> > + ad.u.inode = inode;
> > +
> > + return avc_has_perm(&selinux_state,
> > + tsec->sid,
> > + isec->sid,
> > + isec->sclass,
> > + ANON_INODE__CREATE,
> > + &ad);
> > +}
>
> --
> paul moore
> www.paul-moore.com
