[REVIEW][PATCH 1/4] vfs: Don't allow overwriting mounts in the current mount namespace

[Eric W. Biederman]

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@xxxxxxxxxx> 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.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/namei.c |   25 +++++++++++++++++++++++++
 1 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 645268f23eb6..df7bd6e9c7b6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3547,6 +3547,20 @@ void dentry_unhash(struct dentry *dentry)
 	spin_unlock(&dentry->d_lock);
 }
 
+static bool covered(struct vfsmount *mnt, struct dentry *dentry)
+{
+	/* test to see if a dentry is covered with a mount in
+	 * the current mount namespace.
+	 */
+	bool is_covered;
+
+	rcu_read_lock();
+	is_covered = d_mountpoint(dentry) && __lookup_mnt(mnt, dentry, 1);
+	rcu_read_unlock();
+
+	return is_covered;
+}
+
 int vfs_rmdir(struct inode *dir, struct dentry *dentry)
 {
 	int error = may_delete(dir, dentry, 1);
@@ -3622,6 +3636,9 @@ retry:
 		error = -ENOENT;
 		goto exit3;
 	}
+	error = -EBUSY;
+	if (covered(nd.path.mnt, dentry))
+		goto exit3;
 	error = security_path_rmdir(&nd.path, dentry);
 	if (error)
 		goto exit3;
@@ -3716,6 +3733,9 @@ retry:
 		inode = dentry->d_inode;
 		if (!inode)
 			goto slashes;
+		error = -EBUSY;
+		if (covered(nd.path.mnt, dentry))
+			goto exit2;
 		ihold(inode);
 		error = security_path_unlink(&nd.path, dentry);
 		if (error)
@@ -4164,6 +4184,11 @@ retry:
 	error = -ENOTEMPTY;
 	if (new_dentry == trap)
 		goto exit5;
+	error = -EBUSY;
+	if (covered(oldnd.path.mnt, old_dentry))
+		goto exit5;
+	if (covered(newnd.path.mnt, new_dentry))
+		goto exit5;
 
 	error = security_path_rename(&oldnd.path, old_dentry,
 				     &newnd.path, new_dentry);
-- 
1.7.5.4

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