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From 743848de574b660972f457c28c02cbb19c8aa439 Mon Sep 17 00:00:00 2001
From: "T.kabe" <kabe@>
Date: Fri, 3 Mar 2017 17:06:44 +0900
Subject: [PATCH 4/4] vfs: Lazily remove mounts on unlinked files and directories.

[upstream commit 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe]
[upstream commit 7af1364ffa64db61e386628594836e13d2ef04b5]

commit 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe
Author: Eric W. Biederman <ebiederman@twitter.com>
Date:   Tue Oct 1 18:33:48 2013 -0700

    vfs: Lazily remove mounts on unlinked files and directories.

    With the introduction of mount namespaces and bind mounts it became
    possible to access files and directories that on some paths are mount
    points but are not mount points on other paths.  It is very confusing
    when rm -rf somedir returns -EBUSY simply because somedir is mounted
    somewhere else.  With the addition of user namespaces allowing
    unprivileged mounts this condition has gone from annoying to allowing
    a DOS attack on other users in the system.

    The possibility for mischief is removed by updating the vfs to support
    rename, unlink and rmdir on a dentry that is a mountpoint and by
    lazily unmounting mountpoints on deleted dentries.

    In particular this change allows rename, unlink and rmdir system calls
    on a dentry without a mountpoint in the current mount namespace to
    succeed, and it allows rename, unlink, and rmdir performed on a
    distributed filesystem to update the vfs cache even if when there is a
    mount in some namespace on the original dentry.

    There are two common patterns of maintaining mounts: Mounts on trusted
    paths with the parent directory of the mount point and all ancestory
    directories up to / owned by root and modifiable only by root
    (i.e. /media/xxx, /dev, /dev/pts, /proc, /sys, /sys/fs/cgroup/{cpu,
    cpuacct, ...}, /usr, /usr/local).  Mounts on unprivileged directories
    maintained by fusermount.

    In the case of mounts in trusted directories owned by root and
    modifiable only by root the current parent directory permissions are
    sufficient to ensure a mount point on a trusted path is not removed
    or renamed by anyone other than root, even if there is a context
    where the there are no mount points to prevent this.

    In the case of mounts in directories owned by less privileged users
    races with users modifying the path of a mount point are already a
    danger.  fusermount already uses a combination of chdir,
    /proc/<pid>/fd/NNN, and UMOUNT_NOFOLLOW to prevent these races.  The
    removable of global rename, unlink, and rmdir protection really adds
    nothing new to consider only a widening of the attack window, and
    fusermount is already safe against unprivileged users modifying the
    directory simultaneously.

    In principle for perfect userspace programs returning -EBUSY for
    unlink, rmdir, and rename of dentires that have mounts in the local
    namespace is actually unnecessary.  Unfortunately not all userspace
    programs are perfect so retaining -EBUSY for unlink, rmdir and rename
    of dentries that have mounts in the current mount namespace plays an
    important role of maintaining consistency with historical behavior and
    making imperfect userspace applications hard to exploit.

    v2: Remove spurious old_dentry.
    v3: Optimized shrink_submounts_and_drop
        Removed unsued afs label
    v4: Simplified the changes to check_submounts_and_drop
        Do not rename check_submounts_and_drop shrink_submounts_and_drop
        Document what why we need atomicity in check_submounts_and_drop
        Rely on the parent inode mutex to make d_revalidate and d_invalidate
        an atomic unit.
    v5: Refcount the mountpoint to detach in case of simultaneous
        renames.

    Reviewed-by: Miklos Szeredi <miklos@szeredi.hu>
    Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

commit 7af1364ffa64db61e386628594836e13d2ef04b5
Author: Eric W. Biederman <ebiederm@xmission.com>
Date:   Fri Oct 4 19:15:13 2013 -0700

    vfs: Don't allow overwriting mounts in the current mount namespace

    In preparation for allowing mountpoints to be renamed and unlinked
    in remote filesystems and in other mount namespaces test if on a dentry
    there is a mount in the local mount namespace before allowing it to
    be renamed or unlinked.

    The primary motivation here are old versions of fusermount unmount
    which is not safe if the a path can be renamed or unlinked while it is
    verifying the mount is safe to unmount.  More recent versions are simpler
    and safer by simply using UMOUNT_NOFOLLOW when unmounting a mount
    in a directory owned by an arbitrary user.

    Miklos Szeredi <miklos@szeredi.hu> reports this is approach is good
    enough to remove concerns about new kernels mixed with old versions
    of fusermount.

    A secondary motivation for restrictions here is that it removing empty
    directories that have non-empty mount points on them appears to
    violate the rule that rmdir can not remove empty directories.  As
    Linus Torvalds pointed out this is useful for programs (like git) that
    test if a directory is empty with rmdir.

    Therefore this patch arranges to enforce the existing mount point
    semantics for local mount namespace.

    v2: Rewrote the test to be a drop in replacement for d_mountpoint
    v3: Use bool instead of int as the return type of is_local_mountpoint

    Reviewed-by: Miklos Szeredi <miklos@szeredi.hu>
    Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/dcache.c    | 69 +++++++++++++++++++++++++++++++---------------------------
 fs/mount.h     |  9 ++++++++
 fs/namei.c     | 16 +++++++++-----
 fs/namespace.c | 35 +++++++++++++++++++++++++++++
 4 files changed, 91 insertions(+), 38 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 5dabe0e..a3e9e7a 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1285,36 +1285,38 @@ void shrink_dcache_parent(struct dentry *parent)
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
 
-static enum d_walk_ret check_and_collect(void *_data, struct dentry *dentry)
+struct detach_data {
+	struct select_data select;
+	struct dentry *mountpoint;
+};
+static enum d_walk_ret detach_and_collect(void *_data, struct dentry *dentry)
 {
-	struct select_data *data = _data;
-
-	if (d_mountpoint(dentry)) {
-		data->found = -EBUSY;
-		return D_WALK_QUIT;
-	}
+	struct detach_data *data = _data;
 
-	return select_collect(_data, dentry);
-}
+ 	if (d_mountpoint(dentry)) {
+		__dget_dlock(dentry);
+		data->mountpoint = dentry;
+ 		return D_WALK_QUIT;
+ 	}
+	return select_collect(&data->select, dentry);
+ }
 
 static void check_and_drop(void *_data)
 {
-	struct select_data *data = _data;
+	struct detach_data *data = _data;
 
-	if (d_mountpoint(data->start))
-		data->found = -EBUSY;
-	if (!data->found)
-		__d_drop(data->start);
+	if (!data->mountpoint && !data->select.found)
+		__d_drop(data->select.start);
 }
 
 /**
- * check_submounts_and_drop - prune dcache, check for submounts and drop
+ * check_submounts_and_drop - detach submounts, prune dcache, and drop
  *
- * All done as a single atomic operation relative to has_unlinked_ancestor().
- * Returns 0 if successfully unhashed @parent.  If there were submounts then
- * return -EBUSY.
+ * The final d_drop is done as an atomic operation relative to
+ * rename_lock ensuring there are no races with d_set_mounted.  This
+ * ensures there are no unhashed dentries on the path to a mountpoint.
  *
- * @dentry: dentry to prune and drop
+ * @dentry: dentry to detach, prune and drop
  */
 int check_submounts_and_drop(struct dentry *dentry)
 {
@@ -1327,19 +1329,24 @@ int check_submounts_and_drop(struct dentry *dentry)
 	}
 
 	for (;;) {
-		struct select_data data;
+		struct detach_data data;
 
-		INIT_LIST_HEAD(&data.dispose);
-		data.start = dentry;
-		data.found = 0;
+		data.mountpoint = NULL;
+		INIT_LIST_HEAD(&data.select.dispose);
+		data.select.start = dentry;
+		data.select.found = 0;
 
-		d_walk(dentry, &data, check_and_collect, check_and_drop);
-		ret = data.found;
+		d_walk(dentry, &data, detach_and_collect, check_and_drop);
 
-		if (!list_empty(&data.dispose))
-			shrink_dentry_list(&data.dispose);
+		if (data.select.found)
+			shrink_dentry_list(&data.select.dispose);
 
-		if (ret <= 0)
+		if (data.mountpoint) {
+			detach_mounts(data.mountpoint);
+			dput(data.mountpoint);
+		}
+
+		if (!data.mountpoint && !data.select.found)
 			break;
 
 		cond_resched();
@@ -2554,10 +2561,8 @@ static struct dentry *__d_unalias(struct inode *inode,
 		goto out_err;
 	m2 = &alias->d_parent->d_inode->i_mutex;
 out_unalias:
-	if (likely(!d_mountpoint(alias))) {
-		__d_move(alias, dentry, false);
-		ret = alias;
-	}
+	__d_move(alias, dentry, false);
+	ret = alias;
 out_err:
 	spin_unlock(&inode->i_lock);
 	if (m2)
diff --git a/fs/mount.h b/fs/mount.h
index 9959119..a373c86 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -107,3 +107,12 @@ struct proc_mounts {
 #define proc_mounts(p) (container_of((p), struct proc_mounts, m))
 
 extern const struct seq_operations mounts_op;
+
+extern bool __is_local_mountpoint(struct dentry *dentry);
+static inline bool is_local_mountpoint(struct dentry *dentry)
+{
+	if (!d_mountpoint(dentry))
+		return false;
+
+	return __is_local_mountpoint(dentry);
+}
diff --git a/fs/namei.c b/fs/namei.c
index 872e5e5..ef70aa8 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3691,8 +3691,8 @@ int vfs_rmdir(struct inode *dir, struct dentry *dentry)
 	mutex_lock(&dentry->d_inode->i_mutex);
 
 	error = -EBUSY;
-	if (d_mountpoint(dentry))
-		goto out;
+ 	if (is_local_mountpoint(dentry))
+ 		goto out;
 
 	error = security_inode_rmdir(dir, dentry);
 	if (error)
@@ -3705,6 +3705,7 @@ int vfs_rmdir(struct inode *dir, struct dentry *dentry)
 
 	dentry->d_inode->i_flags |= S_DEAD;
 	dont_mount(dentry);
+	detach_mounts(dentry);
 
 out:
 	mutex_unlock(&dentry->d_inode->i_mutex);
@@ -3806,7 +3807,7 @@ int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegate
 		return -EPERM;
 
 	mutex_lock(&target->i_mutex);
-	if (d_mountpoint(dentry))
+	if (is_local_mountpoint(dentry))
 		error = -EBUSY;
 	else {
 		error = security_inode_unlink(dir, dentry);
@@ -3815,8 +3816,10 @@ int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegate
 			if (error)
 				goto out;
 			error = dir->i_op->unlink(dir, dentry);
-			if (!error)
+			if (!error) {
 				dont_mount(dentry);
+				detach_mounts(dentry);
+			}
 		}
 	}
 out:
@@ -4254,8 +4257,8 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 		mutex_lock(&target->i_mutex);
 
 	error = -EBUSY;
-	if (d_mountpoint(old_dentry) || d_mountpoint(new_dentry))
-		goto out;
+ 	if (is_local_mountpoint(old_dentry) || is_local_mountpoint(new_dentry))
+ 		goto out;
 
 	if (max_links && new_dir != old_dir) {
 		error = -EMLINK;
@@ -4292,6 +4295,7 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 		if (is_dir)
 			target->i_flags |= S_DEAD;
 		dont_mount(new_dentry);
+		detach_mounts(new_dentry);
 	}
 	if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE)) {
 		if (!(flags & RENAME_EXCHANGE))
diff --git a/fs/namespace.c b/fs/namespace.c
index e48fed3..d633562 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -625,6 +625,41 @@ static struct mountpoint *lookup_mountpoint(struct dentry *dentry)
 	return NULL;
 }
 
+/*
+ * __is_local_mountpoint - Test to see if dentry is a mountpoint in the
+ *                         current mount namespace.
+ *
+ * The common case is dentries are not mountpoints at all and that
+ * test is handled inline.  For the slow case when we are actually
+ * dealing with a mountpoint of some kind, walk through all of the
+ * mounts in the current mount namespace and test to see if the dentry
+ * is a mountpoint.
+ *
+ * The mount_hashtable is not usable in the context because we
+ * need to identify all mounts that may be in the current mount
+ * namespace not just a mount that happens to have some specified
+ * parent mount.
+ */
+bool __is_local_mountpoint(struct dentry *dentry)
+{
+	struct mnt_namespace *ns = current->nsproxy->mnt_ns;
+	struct mount *mnt;
+	bool is_covered = false;
+
+	if (!d_mountpoint(dentry))
+		goto out;
+
+	down_read(&namespace_sem);
+	list_for_each_entry(mnt, &ns->list, mnt_list) {
+		is_covered = (mnt->mnt_mountpoint == dentry);
+		if (is_covered)
+			break;
+	}
+	up_read(&namespace_sem);
+out:
+	return is_covered;
+}
+
 static struct mountpoint *new_mountpoint(struct dentry *dentry)
 {
 	struct list_head *chain = mountpoint_hashtable + hash(NULL, dentry);
-- 
1.8.3.1