[RFC PATCH v2] libselinux: add selinux_unshare() and is_selinux_unshared()

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

RFC only, this demonstrates how to implement unsharing of the SELinux
namespace using the lsm_set_self_attr(LSM_ATTR_UNSHARE, ...) system
call and how to implement detection of whether one is in an unshared
SELinux namespace that has not yet been fully initialized using
the lsm_get_self_attr(LSM_ATTR_UNSHARE, ...) system call.

Provide a selinux_unshare() wrapper for
lsm_set_self_attr(LSM_ATTR_UNSHARE, ...) and other required processing
when unsharing the SELinux namespace, and an is_selinux_unshared()
wrapper for lsm_get_self_attr(LSM_ATTR_UNSHARE, ...) for detecting
whether one is in an unshared SELinux namespace that has not yet
been fully initialized.

Add a selinux_unshare() interface to unshare the SELinux namespace, and
a unshareselinux utility to run a shell or command in its own SELinux
and mount namespaces. The selinux_unshare() interface expects the caller
to have already unshared its mount namespace and created a
MS_REC|MS_PRIVATE mount of / prior to invoking it so that it can freely
modify the selinuxfs mount as needed by the unshare operation. The
unshareselinux utility demonstrates how to do this prior to calling
selinux_unshare(). Upon a successful return from selinux_unshare(),
the SELinux namespace will be unshared and there will be no selinuxfs
mount on /sys/fs/selinux.  The caller can then proceed to mount a new
selinuxfs filesystem private to the new namespace, load a policy,
set enforcing mode, etc, as is commonly handled by init/systemd during
boot.

Add an is_selinux_unshared() interface to detect whether one is in
an unshared SELinux namespace that has not yet been fully initialized
(i.e. no policy loaded yet), and a selinuxunshared utility to use
it from a shell or script.

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
---
 libselinux/include/selinux/selinux.h |  22 ++++++
 libselinux/src/libselinux.map        |   6 ++
 libselinux/src/unshare.c             | 114 +++++++++++++++++++++++++++
 libselinux/utils/.gitignore          |   2 +
 libselinux/utils/selinuxunshared.c   |  24 ++++++
 libselinux/utils/unshareselinux.c    |  42 ++++++++++
 6 files changed, 210 insertions(+)
 create mode 100644 libselinux/src/unshare.c
 create mode 100644 libselinux/utils/selinuxunshared.c
 create mode 100644 libselinux/utils/unshareselinux.c

diff --git a/libselinux/include/selinux/selinux.h b/libselinux/include/selinux/selinux.h
index b1431e5d..4cac6283 100644
--- a/libselinux/include/selinux/selinux.h
+++ b/libselinux/include/selinux/selinux.h
@@ -760,6 +760,28 @@ extern int selinux_lsetfilecon_default(const char *path);
  */
 extern void selinux_reset_config(void);
 
+/*
+ * Unshare the SELinux namespace.
+ * Prior to invoking this API, the caller must have unshared the
+ * mount namespace (CLONE_NEWNS) and mounted a MS_REC|MS_PRIVATE
+ * / filesystem so that selinux_unshare() can freely modify any
+ * existing selinuxfs mount as needed for the unshare.
+ * Returns 0 on success, in which case the SELinux namespace has
+ * been unshared and any old selinuxfs mount will have been unmounted.
+ * The caller can then proceed to mount a new selinuxfs filesystem
+ * for the new namespace, load a policy, set enforcing mode, etc.
+ */
+extern int selinux_unshare(void);
+
+/*
+ * Returns 1 if the SELinux namespace was unshared and has not
+ * yet been fully initialized (i.e. policy not yet loaded).
+ * Returns 0 if the SELinux namespace was either not unshared or
+ * has been fully initialized.
+ * Returns -1 if not supported.
+ */
+extern int is_selinux_unshared(void);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/libselinux/src/libselinux.map b/libselinux/src/libselinux.map
index ab002f01..77f5790c 100644
--- a/libselinux/src/libselinux.map
+++ b/libselinux/src/libselinux.map
@@ -262,3 +262,9 @@ LIBSELINUX_3.9 {
   global:
     context_to_str;
 } LIBSELINUX_3.8;
+
+LIBSELINUX_4.0 {
+  global:
+    selinux_unshare;
+    is_selinux_unshared;
+} LIBSELINUX_3.9;
diff --git a/libselinux/src/unshare.c b/libselinux/src/unshare.c
new file mode 100644
index 00000000..16edb2c0
--- /dev/null
+++ b/libselinux/src/unshare.c
@@ -0,0 +1,114 @@
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#include <sched.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sys/mount.h>
+#include <sys/vfs.h>
+#include <sys/statvfs.h>
+#include <sys/syscall.h>
+#include <linux/lsm.h>
+
+#include "selinux_internal.h"
+#include "policy.h"
+
+#ifndef LSM_ATTR_UNSHARE
+#define LSM_ATTR_UNSHARE 106
+#endif
+
+#ifndef __NR_lsm_get_self_attr
+#define __NR_lsm_get_self_attr 459
+#endif
+
+#ifndef __NR_lsm_set_self_attr
+#define __NR_lsm_set_self_attr 460
+#endif
+
+#ifndef HAVE_LSM_SET_SELF_ATTR
+#define HAVE_LSM_SET_SELF_ATTR 1
+static int lsm_set_self_attr(unsigned int attr, struct lsm_ctx *ctx,
+			     uint32_t size, uint32_t flags)
+{
+	return syscall(__NR_lsm_set_self_attr, attr, ctx, size, flags);
+}
+#endif
+
+#ifndef HAVE_LSM_GET_SELF_ATTR
+#define HAVE_LSM_GET_SELF_ATTR 1
+static int lsm_get_self_attr(unsigned int attr, struct lsm_ctx *ctx,
+			     uint32_t *size, uint32_t flags)
+{
+	return syscall(__NR_lsm_get_self_attr, attr, ctx, size, flags);
+}
+#endif
+
+/*
+ * Precondition: caller must have already done unshare(CLONE_NEWNS) or
+ * been created via clone(CLONE_NEWNS) and mounted a MS_REC|MS_PRIVATE
+ * / filesystem so that any pre-existing selinuxfs mount can be
+ * modified freely by selinux_unshare(). See ../utils/unshareselinux.c
+ * for an example.
+ */
+int selinux_unshare(void)
+{
+	struct lsm_ctx ctx;
+	int ret;
+
+	ctx.id = LSM_ID_SELINUX;
+	ctx.flags = 0;
+	ctx.len = sizeof(ctx);
+	ctx.ctx_len = 0;
+
+	/* Unshare the SELinux namespace */
+	ret = lsm_set_self_attr(LSM_ATTR_UNSHARE, &ctx, sizeof(ctx), 0);
+	if (ret < 0)
+		return -1;
+
+	/* Unmount the selinuxfs which refers to the old/parent namespace */
+	ret = umount(SELINUXMNT);
+	if (ret < 0)
+		return ret;
+
+	/*
+	 * Caller is responsible for mounting new selinuxfs, loading policy,
+	 * setting enforcing mode, etc.
+	 */
+
+	return 0;
+}
+
+struct selinux_ctx {
+	struct lsm_ctx lsmctx;
+	char unshared;
+};
+
+/*
+ * Returns 1 if the SELinux namespace was unshared and has not
+ * yet been fully initialized (i.e. policy not yet loaded).
+ * Returns 0 if the SELinux namespace was either not unshared or
+ * has been fully initialized.
+ * Returns -1 on any other case, e.g. operation not supported.
+ */
+int is_selinux_unshared(void)
+{
+	struct selinux_ctx ctx;
+	uint32_t size = sizeof(ctx);
+	int ret;
+
+	ctx.lsmctx.id = LSM_ID_SELINUX;
+	ctx.lsmctx.flags = 0;
+	ctx.lsmctx.len = sizeof(ctx);
+	ctx.lsmctx.ctx_len = 0;
+
+	ret = lsm_get_self_attr(LSM_ATTR_UNSHARE, (struct lsm_ctx *)&ctx,
+				&size, LSM_FLAG_SINGLE);
+	if (ret < 0)
+		return ret;
+	return ctx.unshared;
+}
diff --git a/libselinux/utils/.gitignore b/libselinux/utils/.gitignore
index 2e10b14f..ffd2bc8e 100644
--- a/libselinux/utils/.gitignore
+++ b/libselinux/utils/.gitignore
@@ -30,3 +30,5 @@ setfilecon
 togglesebool
 selinux_check_access
 validatetrans
+unshareselinux
+selinuxunshared
diff --git a/libselinux/utils/selinuxunshared.c b/libselinux/utils/selinuxunshared.c
new file mode 100644
index 00000000..965f7ba3
--- /dev/null
+++ b/libselinux/utils/selinuxunshared.c
@@ -0,0 +1,24 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <selinux/selinux.h>
+
+int main(int argc, char **argv)
+{
+	int ret;
+
+	if (argc != 1) {
+		fprintf(stderr, "usage: %s\n", argv[0]);
+		exit(-1);
+	}
+
+	ret = is_selinux_unshared();
+	if (ret < 0) {
+		perror(argv[0]);
+		exit(-1);
+	}
+
+	printf("%d\n", ret);
+
+	exit(!ret);
+}
diff --git a/libselinux/utils/unshareselinux.c b/libselinux/utils/unshareselinux.c
new file mode 100644
index 00000000..b396b4fe
--- /dev/null
+++ b/libselinux/utils/unshareselinux.c
@@ -0,0 +1,42 @@
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sys/mount.h>
+#include <selinux/selinux.h>
+
+int main(int argc, char **argv)
+{
+	int ret;
+
+	ret = unshare(CLONE_NEWNS);
+	if (ret < 0) {
+		perror("unshare(CLONE_NEWNS)");
+		exit(1);
+	}
+
+	ret = mount("none", "/", NULL, MS_REC | MS_PRIVATE, NULL);
+	if (ret < 0) {
+		perror("mount(/)");
+		exit(1);
+	}
+
+	ret = selinux_unshare();
+	if (ret < 0) {
+		perror("selinux_unshare");
+		exit(1);
+	}
+
+	if (argc < 2) {
+		fprintf(stderr, "usage: %s command args...\n", argv[0]);
+		exit(1);
+	}
+
+	execvp(argv[1], &argv[1]);
+	perror(argv[1]);
+}
-- 
2.51.0