make sure that mntns_install() doesn't end up with referral for root

new flag: LOOKUP_DOWN.  If the starting point is overmounted, cross
into whatever's mounted on top, triggering referrals et.al.

Use that instead of follow_down_one() loop in mntns_install(), handle
errors properly.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

3 files changed, 50 insertions, 7 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 60c0a78ebca7..646db9cf2579 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2252,6 +2252,35 @@ static inline int lookup_last(struct nameidata *nd)
         return walk_component(nd, 0);
 }
 
+static int handle_lookup_down(struct nameidata *nd)
+{
+        struct path path = nd->path;
+        struct inode *inode = nd->inode;
+        unsigned seq = nd->seq;
+        int err;
+
+        if (nd->flags & LOOKUP_RCU) {
+                /*
+                 * don't bother with unlazy_walk on failure - we are
+                 * at the very beginning of walk, so we lose nothing
+                 * if we simply redo everything in non-RCU mode
+                 */
+                if (unlikely(!__follow_mount_rcu(nd, &path, &inode, &seq)))
+                        return -ECHILD;
+        } else {
+                dget(path.dentry);
+                err = follow_managed(&path, nd);
+                if (unlikely(err < 0))
+                        return err;
+                inode = d_backing_inode(path.dentry);
+                seq = 0;
+        }
+        path_to_nameidata(&path, nd);
+        nd->inode = inode;
+        nd->seq = seq;
+        return 0;
+}
+
 /* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
 static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path)
 {
@@ -2260,6 +2289,15 @@ static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path
 
         if (IS_ERR(s))
                 return PTR_ERR(s);
+
+        if (unlikely(flags & LOOKUP_DOWN)) {
+                err = handle_lookup_down(nd);
+                if (unlikely(err < 0)) {
+                        terminate_walk(nd);
+                        return err;
+                }
+        }
+
         while (!(err = link_path_walk(s, nd))
                 && ((err = lookup_last(nd)) > 0)) {
                 s = trailing_symlink(nd);
diff --git a/fs/namespace.c b/fs/namespace.c
index cc1375eff88c..0886ef28bff6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3465,8 +3465,9 @@ static void mntns_put(struct ns_common *ns)
 static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 {
         struct fs_struct *fs = current->fs;
-        struct mnt_namespace *mnt_ns = to_mnt_ns(ns);
+        struct mnt_namespace *mnt_ns = to_mnt_ns(ns), *old_mnt_ns;
         struct path root;
+        int err;
 
         if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
             !ns_capable(current_user_ns(), CAP_SYS_CHROOT) ||
@@ -3477,15 +3478,18 @@ static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
                 return -EINVAL;
 
         get_mnt_ns(mnt_ns);
-        put_mnt_ns(nsproxy->mnt_ns);
+        old_mnt_ns = nsproxy->mnt_ns;
         nsproxy->mnt_ns = mnt_ns;
 
         /* Find the root */
-        root.mnt    = &mnt_ns->root->mnt;
-        root.dentry = mnt_ns->root->mnt.mnt_root;
-        path_get(&root);
-        while(d_mountpoint(root.dentry) && follow_down_one(&root))
-                ;
+        err = vfs_path_lookup(mnt_ns->root->mnt.mnt_root, &mnt_ns->root->mnt,
+                                "/", LOOKUP_DOWN, &root);
+        if (err) {
+                /* revert to old namespace */
+                nsproxy->mnt_ns = old_mnt_ns;
+                put_mnt_ns(mnt_ns);
+                return err;
+        }
 
         /* Update the pwd and root */
         set_fs_pwd(fs, &root);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index f29abda31e6d..8b4794e83196 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -44,6 +44,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_JUMPED           0x1000
 #define LOOKUP_ROOT             0x2000
 #define LOOKUP_EMPTY            0x4000
+#define LOOKUP_DOWN             0x8000
 
 extern int path_pts(struct path *path);