Re: [PATCH v12 3/4] selinux: teach SELinux about anonymous inodes

[Lokesh Gidra <lokeshgidra@google.com>]

Thanks a lot Paul for the reviewing this patch.

On Mon, Nov 9, 2020 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
>
> On Fri, Nov 6, 2020 at 10:56 AM 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            | 53 +++++++++++++++++++++++++++++
> >  security/selinux/include/classmap.h |  2 ++
> >  2 files changed, 55 insertions(+)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 6b1826fc3658..1c0adcdce7a8 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -2927,6 +2927,58 @@ 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);
> > +               isec->sclass = context_isec->sclass;
> > +               isec->sid = context_isec->sid;
>
> I suppose this isn't a major concern given the limited usage at the
> moment, but I wonder if it would be a good idea to make sure the
> context_inode's SELinux label is valid before we assign it to the
> anonymous inode?  If it is invalid, what should we do?  Do we attempt
> to (re)validate it?  Do we simply fallback to the transition approach?
>
Frankly, I'm not too familiar with SELinux. Originally this patch
series was developed by Daniel, in consultation with Stephen Smalley.
In my (probably naive) opinion we should fallback to transition
approach. But I'd request you to tell me if this needs to be addressed
now, and if so then what's the right approach.

If the decision is to address this now, then what's the best way to
check the SELinux label validity?

> > +       } 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,
> > +                           FILE__CREATE,
>
> I believe you want to use ANON_INODE__CREATE here instead of FILE__CREATE, yes?

ANON_INODE__CREATE definitely seems more appropriate. I'll change it
in the next revision.
>
> This brings up another question, and requirement - what testing are
> you doing for this patchset?  We require that new SELinux kernel
> functionality includes additions to the SELinux test suite to help
> verify the functionality.  I'm also *strongly* encouraging that new
> contributions come with updates to The SELinux Notebook.  If you are
> unsure about what to do for either, let us know and we can help get
> you started.
>
> * https://github.com/SELinuxProject/selinux-testsuite
> * https://github.com/SELinuxProject/selinux-notebook
>
I'd definitely need help with both of these. Kindly guide how to proceed.

> > +                           &ad);
> > +}
> > +
> >  static int selinux_inode_create(struct inode *dir, struct dentry *dentry, umode_t mode)
> >  {
> >         return may_create(dir, dentry, SECCLASS_FILE);
> > @@ -6992,6 +7044,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
> >
> >         LSM_HOOK_INIT(inode_free_security, selinux_inode_free_security),
> >         LSM_HOOK_INIT(inode_init_security, selinux_inode_init_security),
> > +       LSM_HOOK_INIT(inode_init_security_anon, selinux_inode_init_security_anon),
> >         LSM_HOOK_INIT(inode_create, selinux_inode_create),
> >         LSM_HOOK_INIT(inode_link, selinux_inode_link),
> >         LSM_HOOK_INIT(inode_unlink, selinux_inode_unlink),
> > diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> > index 40cebde62856..ba2e01a6955c 100644
> > --- a/security/selinux/include/classmap.h
> > +++ b/security/selinux/include/classmap.h
> > @@ -249,6 +249,8 @@ struct security_class_mapping secclass_map[] = {
> >           {"open", "cpu", "kernel", "tracepoint", "read", "write"} },
> >         { "lockdown",
> >           { "integrity", "confidentiality", NULL } },
> > +       { "anon_inode",
> > +         { COMMON_FILE_PERMS, NULL } },
> >         { NULL }
> >    };
> >
>
> --
> paul moore
> www.paul-moore.com