From: Serge E. Hallyn <serue@us.ibm.com>
Subject: [PATCH 6/8] user ns: implement shared mounts

Implement shared-ns mounts, which allow containers in different user
namespaces to share mounts.  Without this, containers can obviously
never even be started.

Here is a sample smount.c (based on Miklos' version) which only
does a bind mount of arg1 onto arg2, but making the destination
a shared-ns 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_SHARE_NS | 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        |   30 ++++++++++++++++++++++++------
 fs/pnode.h            |    1 +
 include/linux/fs.h    |    1 +
 include/linux/mount.h |    1 +
 include/linux/sched.h |    2 ++
 5 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index f85dd73..9cf8463 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -235,7 +235,14 @@ static struct vfsmount *clone_mnt(struct
 					int flag)
 {
 	struct super_block *sb = old->mnt_sb;
-	struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname);
+	struct vfsmount *mnt;
+	
+	if (!(old->mnt_flags & MNT_SHARE_NS)) {
+		if (old->mnt_user_ns != current->nsproxy->user_ns)
+			return ERR_PTR(-EPERM);
+	}
+
+	mnt = alloc_vfsmnt(old->mnt_devname);
 
 	if (mnt) {
 		mnt->mnt_flags = old->mnt_flags;
@@ -258,6 +265,10 @@ static struct vfsmount *clone_mnt(struct
 		}
 		if (flag & CL_MAKE_SHARED)
 			set_mnt_shared(mnt);
+		if (flag & CL_SHARE_NS)
+			mnt->mnt_flags |= MNT_SHARE_NS;
+		else
+			mnt->mnt_flags &= ~MNT_SHARE_NS;
 
 		/* stick the duplicate mount on the same expiry list
 		 * as the original if that was on one */
@@ -369,6 +380,7 @@ static int show_vfsmnt(struct seq_file *
 		{ MNT_NOSUID, ",nosuid" },
 		{ MNT_NODEV, ",nodev" },
 		{ MNT_NOEXEC, ",noexec" },
+		{ MNT_SHARE_NS, ",share_userns" },
 		{ MNT_NOATIME, ",noatime" },
 		{ MNT_NODIRATIME, ",nodiratime" },
 		{ MNT_RELATIME, ",relatime" },
@@ -903,11 +915,14 @@ 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_SHARE_NS : 0);
+
 	if (err)
 		return err;
 	if (!old_name || !*old_name)
@@ -926,9 +941,9 @@ 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 || IS_ERR(mnt)) {
 		err = mnt ? PTR_ERR(mnt) : -ENOMEM;
@@ -1415,9 +1430,11 @@ long do_mount(char *dev_name, char *dir_
 		mnt_flags |= MNT_NODIRATIME;
 	if (flags & MS_RELATIME)
 		mnt_flags |= MNT_RELATIME;
+	if (flags & MS_SHARE_NS)
+		mnt_flags |= MNT_SHARE_NS;
 
 	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
-		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME);
+		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME | MS_SHARE_NS);
 
 	/* ... and get the mountpoint */
 	retval = path_lookup(dir_name, LOOKUP_FOLLOW, &nd);
@@ -1432,7 +1449,8 @@ 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,
+						 mnt_flags & MNT_SHARE_NS);
 	else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
 		retval = do_change_type(&nd, flags);
 	else if (flags & MS_MOVE)
diff --git a/fs/pnode.h b/fs/pnode.h
index d45bd8e..eb62f4c 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -22,6 +22,7 @@ #define CL_SLAVE     		0x02
 #define CL_COPY_ALL 		0x04
 #define CL_MAKE_SHARED 		0x08
 #define CL_PROPAGATION 		0x10
+#define CL_SHARE_NS 		0x20
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e7268d2..bb801cb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -121,6 +121,7 @@ #define MS_PRIVATE	(1<<18)	/* change to 
 #define MS_SLAVE	(1<<19)	/* change to slave */
 #define MS_SHARED	(1<<20)	/* change to shared */
 #define MS_RELATIME	(1<<21)	/* Update atime relative to mtime/ctime. */
+#define MS_SHARE_NS	(1<<22) /* ignore user namespaces for permission */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)
 
diff --git a/include/linux/mount.h b/include/linux/mount.h
index acdeca7..28e0964 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -35,6 +35,7 @@ #define MNT_SHRINKABLE	0x100
 #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_SHARE_NS	0x4000  /* ignore user namespaces for permission */
 
 struct vfsmount {
 	struct list_head mnt_hash;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 450fc39..73df38c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1599,6 +1599,8 @@ static inline int task_mnt_same_uidns(st
 {
 	if (tsk->nsproxy == init_task.nsproxy)
 		return 1;
+ 	if (mnt->mnt_flags & MNT_SHARE_NS)
+ 		return 1;
 	if (mnt->mnt_user_ns == tsk->nsproxy->user_ns)
 		return 1;
 	return 0;
-- 
1.4.1

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