vfs: Only support slave subtrees across different user namespaces

Sharing mount subtress with mount namespaces created by unprivileged
users allows unprivileged mounts created by unprivileged users to
propagate to mount namespaces controlled by privileged users.

Prevent nasty consequences by changing shared subtrees to slave
subtress when an unprivileged users creates a new mount namespace.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

2 files changed, 9 insertions, 3 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 207c7ba84ad3..4dfcaf05d17c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -786,7 +786,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
         if (!mnt)
                 return ERR_PTR(-ENOMEM);
 
-        if (flag & (CL_SLAVE | CL_PRIVATE))
+        if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
                 mnt->mnt_group_id = 0; /* not a peer of original */
         else
                 mnt->mnt_group_id = old->mnt_group_id;
@@ -807,7 +807,8 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
         list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
         br_write_unlock(&vfsmount_lock);
 
-        if (flag & CL_SLAVE) {
+        if ((flag & CL_SLAVE) ||
+            ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
                 list_add(&mnt->mnt_slave, &old->mnt_slave_list);
                 mnt->mnt_master = old;
                 CLEAR_MNT_SHARED(mnt);
@@ -2331,6 +2332,7 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
         struct mount *p, *q;
         struct mount *old = mnt_ns->root;
         struct mount *new;
+        int copy_flags;
 
         new_ns = alloc_mnt_ns(user_ns);
         if (IS_ERR(new_ns))
@@ -2338,7 +2340,10 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
 
         down_write(&namespace_sem);
         /* First pass: copy the tree topology */
-        new = copy_tree(old, old->mnt.mnt_root, CL_COPY_ALL | CL_EXPIRE);
+        copy_flags = CL_COPY_ALL | CL_EXPIRE;
+        if (user_ns != mnt_ns->user_ns)
+                copy_flags |= CL_SHARED_TO_SLAVE;
+        new = copy_tree(old, old->mnt.mnt_root, copy_flags);
         if (IS_ERR(new)) {
                 up_write(&namespace_sem);
                 free_mnt_ns(new_ns);
diff --git a/fs/pnode.h b/fs/pnode.h
index 65c60979d541..19b853a3445c 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -22,6 +22,7 @@
 #define CL_COPY_ALL             0x04
 #define CL_MAKE_SHARED          0x08
 #define CL_PRIVATE              0x10
+#define CL_SHARED_TO_SLAVE      0x20
 
 static inline void set_mnt_shared(struct mount *mnt)
 {