capabilities: factor out cap_bprm_set_creds privileged root

Factor out the case of privileged root from the function
cap_bprm_set_creds() to make the latter easier to read and analyse.
Suggested-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Acked-by: James Morris <james.l.morris@oracle.com>
Acked-by: Kees Cook <keescook@chromium.org>
Okay-ished-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

security/commoncap.c

@@ -695,33 +695,26 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_c
 	return rc;
 }
 
-/**
- * cap_bprm_set_creds - Set up the proposed credentials for execve().
+/*
+ * handle_privileged_root - Handle case of privileged root
  * @bprm: The execution parameters, including the proposed creds
+ * @has_fcap: Are any file capabilities set?
+ * @effective: Do we have effective root privilege?
+ * @root_uid: This namespace' root UID WRT initial USER namespace
  *
- * Set up the proposed credentials for a new execution context being
- * constructed by execve().  The proposed creds in @bprm->cred is altered,
- * which won't take effect immediately.  Returns 0 if successful, -ve on error.
+ * Handle the case where root is privileged and hasn't been neutered by
+ * SECURE_NOROOT.  If file capabilities are set, they won't be combined with
+ * set UID root and nothing is changed.  If we are root, cap_permitted is
+ * updated.  If we have become set UID root, the effective bit is set.
  */
-int cap_bprm_set_creds(struct linux_binprm *bprm)
+static void handle_privileged_root(struct linux_binprm *bprm, bool has_cap,
+				   bool *effective, kuid_t root_uid)
 {
 	const struct cred *old = current_cred();
 	struct cred *new = bprm->cred;
-	bool effective, has_cap = false, is_setid;
-	int ret;
-	kuid_t root_uid;
-
-	if (WARN_ON(!cap_ambient_invariant_ok(old)))
-		return -EPERM;
-
-	effective = false;
-	ret = get_file_caps(bprm, &effective, &has_cap);
-	if (ret < 0)
-		return ret;
 
-	root_uid = make_kuid(new->user_ns, 0);
-
-	if (!issecure(SECURE_NOROOT)) {
+	if (issecure(SECURE_NOROOT))
+		return;
 	/*
 	 * If the legacy file capability is set, then don't set privs
 	 * for a setuid root binary run by a non-root user.  Do set it
@@ -729,24 +722,51 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 	 */
 	if (has_cap && !uid_eq(new->uid, root_uid) && uid_eq(new->euid, root_uid)) {
 		warn_setuid_and_fcaps_mixed(bprm->filename);
-			goto skip;
+		return;
 	}
 	/*
 	 * To support inheritance of root-permissions and suid-root
 	 * executables under compatibility mode, we override the
 	 * capability sets for the file.
-		 *
-		 * If only the real uid is 0, we do not set the effective bit.
 	 */
 	if (uid_eq(new->euid, root_uid) || uid_eq(new->uid, root_uid)) {
 		/* pP' = (cap_bset & ~0) | (pI & ~0) */
 		new->cap_permitted = cap_combine(old->cap_bset,
 						 old->cap_inheritable);
 	}
+	/*
+	 * If only the real uid is 0, we do not set the effective bit.
+	 */
 	if (uid_eq(new->euid, root_uid))
-			effective = true;
-	}
-skip:
+		*effective = true;
+}
+
+/**
+ * cap_bprm_set_creds - Set up the proposed credentials for execve().
+ * @bprm: The execution parameters, including the proposed creds
+ *
+ * Set up the proposed credentials for a new execution context being
+ * constructed by execve().  The proposed creds in @bprm->cred is altered,
+ * which won't take effect immediately.  Returns 0 if successful, -ve on error.
+ */
+int cap_bprm_set_creds(struct linux_binprm *bprm)
+{
+	const struct cred *old = current_cred();
+	struct cred *new = bprm->cred;
+	bool effective = false, has_cap = false, is_setid;
+	int ret;
+	kuid_t root_uid;
+
+	if (WARN_ON(!cap_ambient_invariant_ok(old)))
+		return -EPERM;
+
+	ret = get_file_caps(bprm, &effective, &has_cap);
+	if (ret < 0)
+		return ret;
+
+	root_uid = make_kuid(new->user_ns, 0);
+
+	handle_privileged_root(bprm, has_cap, &effective, root_uid);
 
 	/* if we have fs caps, clear dangerous personality flags */
 	if (!cap_issubset(new->cap_permitted, old->cap_permitted))