fix copy_tree() regression

Since 3.14 we had copy_tree() get the shadowing wrong - if we had one
vfsmount shadowing another (i.e. if A is a slave of B, C is mounted
on A/foo, then D got mounted on B/foo creating D' on A/foo shadowed
by C), copy_tree() of A would make a copy of D' shadow the the copy of
C, not the other way around.

It's easy to fix, fortunately - just make sure that mount follows
the one that shadows it in mnt_child as well as in mnt_hash, and when
copy_tree() decides to attach a new mount, check if the last child
it has added to the same parent should be shadowing the new one.
And if it should, just use the same logics commit_tree() has - put the
new mount into the hash and children lists right after the one that
should shadow it.

Cc: stable@vger.kernel.org [3.14 and later]
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 24 insertions, 7 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 65af9d0e0d67..be3f6f23a47d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -778,6 +778,20 @@ static void attach_mnt(struct mount *mnt,
         list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
 }
 
+static void attach_shadowed(struct mount *mnt,
+                        struct mount *parent,
+                        struct mount *shadows)
+{
+        if (shadows) {
+                hlist_add_after_rcu(&shadows->mnt_hash, &mnt->mnt_hash);
+                list_add(&mnt->mnt_child, &shadows->mnt_child);
+        } else {
+                hlist_add_head_rcu(&mnt->mnt_hash,
+                                m_hash(&parent->mnt, mnt->mnt_mountpoint));
+                list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
+        }
+}
+
 /*
  * vfsmount lock must be held for write
  */
@@ -796,12 +810,7 @@ static void commit_tree(struct mount *mnt, struct mount *shadows)
 
         list_splice(&head, n->list.prev);
 
-        if (shadows)
-                hlist_add_after_rcu(&shadows->mnt_hash, &mnt->mnt_hash);
-        else
-                hlist_add_head_rcu(&mnt->mnt_hash,
-                                m_hash(&parent->mnt, mnt->mnt_mountpoint));
-        list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
+        attach_shadowed(mnt, parent, shadows);
         touch_mnt_namespace(n);
 }
 
@@ -1474,6 +1483,7 @@ struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
                         continue;
 
                 for (s = r; s; s = next_mnt(s, r)) {
+                        struct mount *t = NULL;
                         if (!(flag & CL_COPY_UNBINDABLE) &&
                             IS_MNT_UNBINDABLE(s)) {
                                 s = skip_mnt_tree(s);
@@ -1495,7 +1505,14 @@ struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
                                 goto out;
                         lock_mount_hash();
                         list_add_tail(&q->mnt_list, &res->mnt_list);
-                        attach_mnt(q, parent, p->mnt_mp);
+                        mnt_set_mountpoint(parent, p->mnt_mp, q);
+                        if (!list_empty(&parent->mnt_mounts)) {
+                                t = list_last_entry(&parent->mnt_mounts,
+                                        struct mount, mnt_child);
+                                if (t->mnt_mp != p->mnt_mp)
+                                        t = NULL;
+                        }
+                        attach_shadowed(q, parent, t);
                         unlock_mount_hash();
                 }
         }