From: Serge E. Hallyn <serue@us.ibm.com>

Implement shared and private userns mounts.  Private userns mounts
are ones which no process in another user namespace can use.

Here is a sample mount program which only does a bind mount of arg1
onto arg2, but making the destination a private userns mount.

int main(int argc, char *argv[])
{
	int type;
	if(argc != 3) {
		fprintf(stderr, "usage: %s src dest", argv[0]);
		return 1;
	}

	fprintf(stdout, "%s %s %s\n", argv[0], argv[1], argv[2]);

	type = MS_PRIV_USERNS | MS_BIND;
	setfsuid(getuid());

	if(mount(argv[1], argv[2], "none", type, "") == -1) {
		perror("mount");
		return 1;
	}
	return 0;
}

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
---
 fs/namespace.c        |   16 ++++++++++++----
 fs/pnode.h            |    1 +
 include/linux/fs.h    |    1 +
 include/linux/mount.h |    1 +
 include/linux/sched.h |    2 ++
 5 files changed, 17 insertions(+), 4 deletions(-)

Index: 2.6.19-rc6-mm2/fs/namespace.c
===================================================================
--- 2.6.19-rc6-mm2.orig/fs/namespace.c
+++ 2.6.19-rc6-mm2/fs/namespace.c
@@ -259,6 +259,8 @@ static struct vfsmount *clone_mnt(struct
 		}
 		if (flag & CL_MAKE_SHARED)
 			set_mnt_shared(mnt);
+		if (flag & CL_PRIV_USERNS)
+			mnt->mnt_flags |= MNT_PRIV_USERNS;
 
 		/* stick the duplicate mount on the same expiry list
 		 * as the original if that was on one */
@@ -370,6 +372,7 @@ static int show_vfsmnt(struct seq_file *
 		{ MNT_NOSUID, ",nosuid" },
 		{ MNT_NODEV, ",nodev" },
 		{ MNT_NOEXEC, ",noexec" },
+		{ MNT_PRIV_USERNS, ",privuserns" },
 		{ MNT_NOATIME, ",noatime" },
 		{ MNT_NODIRATIME, ",nodiratime" },
 		{ 0, NULL }
@@ -901,11 +904,14 @@ static int do_change_type(struct nameida
 /*
  * do loopback mount.
  */
-static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
+static int do_loopback(struct nameidata *nd, char *old_name, int recurse,
+							 int uidns_share)
 {
 	struct nameidata old_nd;
 	struct vfsmount *mnt = NULL;
 	int err = mount_is_safe(nd);
+	int flag = (uidns_share ? CL_PRIV_USERNS : 0);
+
 	if (err)
 		return err;
 	if (!old_name || !*old_name)
@@ -924,9 +930,9 @@ static int do_loopback(struct nameidata 
 
 	err = -ENOMEM;
 	if (recurse)
-		mnt = copy_tree(old_nd.mnt, old_nd.dentry, 0);
+		mnt = copy_tree(old_nd.mnt, old_nd.dentry, flag);
 	else
-		mnt = clone_mnt(old_nd.mnt, old_nd.dentry, 0);
+		mnt = clone_mnt(old_nd.mnt, old_nd.dentry, flag);
 
 	if (!mnt)
 		goto out;
@@ -1409,6 +1415,8 @@ long do_mount(char *dev_name, char *dir_
 		mnt_flags |= MNT_NOATIME;
 	if (flags & MS_NODIRATIME)
 		mnt_flags |= MNT_NODIRATIME;
+	if (flags & MS_PRIV_USERNS)
+		mnt_flags |= MNT_PRIV_USERNS;
 
 	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
 		   MS_NOATIME | MS_NODIRATIME);
@@ -1426,7 +1434,7 @@ long do_mount(char *dev_name, char *dir_
 		retval = do_remount(&nd, flags & ~MS_REMOUNT, mnt_flags,
 				    data_page);
 	else if (flags & MS_BIND)
-		retval = do_loopback(&nd, dev_name, flags & MS_REC);
+		retval = do_loopback(&nd, dev_name, flags & MS_REC, flags & MS_PRIV_USERNS);
 	else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
 		retval = do_change_type(&nd, flags);
 	else if (flags & MS_MOVE)
Index: 2.6.19-rc6-mm2/fs/pnode.h
===================================================================
--- 2.6.19-rc6-mm2.orig/fs/pnode.h
+++ 2.6.19-rc6-mm2/fs/pnode.h
@@ -22,6 +22,7 @@
 #define CL_COPY_ALL 		0x04
 #define CL_MAKE_SHARED 		0x08
 #define CL_PROPAGATION 		0x10
+#define CL_PRIV_USERNS 		0x20
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {
Index: 2.6.19-rc6-mm2/include/linux/fs.h
===================================================================
--- 2.6.19-rc6-mm2.orig/include/linux/fs.h
+++ 2.6.19-rc6-mm2/include/linux/fs.h
@@ -120,6 +120,7 @@ extern int dir_notify_enable;
 #define MS_PRIVATE	(1<<18)	/* change to private */
 #define MS_SLAVE	(1<<19)	/* change to slave */
 #define MS_SHARED	(1<<20)	/* change to shared */
+#define MS_PRIV_USERNS	(1<<21) /* compare user namespaces for permission */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)
 
Index: 2.6.19-rc6-mm2/include/linux/mount.h
===================================================================
--- 2.6.19-rc6-mm2.orig/include/linux/mount.h
+++ 2.6.19-rc6-mm2/include/linux/mount.h
@@ -34,6 +34,7 @@ struct user_namespace;
 #define MNT_SHARED	0x1000	/* if the vfsmount is a shared mount */
 #define MNT_UNBINDABLE	0x2000	/* if the vfsmount is a unbindable mount */
 #define MNT_PNODE_MASK	0x3000	/* propogation flag mask */
+#define MNT_PRIV_USERNS	0x4000  /* compare user namespaces for permission */
 
 struct vfsmount {
 	struct list_head mnt_hash;
Index: 2.6.19-rc6-mm2/include/linux/sched.h
===================================================================
--- 2.6.19-rc6-mm2.orig/include/linux/sched.h
+++ 2.6.19-rc6-mm2/include/linux/sched.h
@@ -1596,6 +1596,8 @@ static inline int task_mnt_same_uid(stru
 {
 	if (tsk->nsproxy == init_task.nsproxy)
 		return 1;
+	if (!(mnt->mnt_flags & MNT_PRIV_USERNS))
+		return 1;
 	if (mnt->mnt_user_ns == tsk->nsproxy->user_ns)
 		return 1;
 	return 0;

-- 
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