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SubjectRe: [PATCH 03/11] vfs: Don't allow overwriting mounts in the current mount namespace
On Sat, Feb 15, 2014 at 01:36:38PM -0800, Eric W. Biederman wrote:
>
> 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
>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
> fs/mount.h | 9 +++++++++
> fs/namei.c | 8 +++++++-
> fs/namespace.c | 35 +++++++++++++++++++++++++++++++++++
> 3 files changed, 51 insertions(+), 1 deletions(-)
>
> diff --git a/fs/mount.h b/fs/mount.h
> index a17458ca6f29..17d913d86add 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -109,3 +109,12 @@ struct proc_mounts {
> #define proc_mounts(p) (container_of((p), struct proc_mounts, m))
>
> extern const struct seq_operations mounts_op;
> +
> +extern int __is_local_mountpoint(struct dentry *dentry);
> +static inline int is_local_mountpoint(struct dentry *dentry)
> +{
> + if (!d_mountpoint(dentry))
> + return 0;
> +
> + return __is_local_mountpoint(dentry);
> +}
> diff --git a/fs/namei.c b/fs/namei.c
> index d580df2e6804..4e6fe16ef488 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3507,6 +3507,8 @@ int vfs_rmdir(struct inode *dir, struct dentry *dentry)
> mutex_lock(&dentry->d_inode->i_mutex);
>
> error = -EBUSY;
> + if (is_local_mountpoint(dentry))
> + goto out;
> if (d_mountpoint(dentry))
> goto out;
>
> @@ -3622,7 +3624,7 @@ 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) || d_mountpoint(dentry))
> error = -EBUSY;
> else {
> error = security_inode_unlink(dir, dentry);
> @@ -4001,6 +4003,8 @@ static int vfs_rename_dir(struct inode *old_dir, struct dentry *old_dentry,
> mutex_lock(&target->i_mutex);
>
> error = -EBUSY;
> + if (is_local_mountpoint(old_dentry) || is_local_mountpoint(new_dentry))
> + goto out;
> if (d_mountpoint(old_dentry) || d_mountpoint(new_dentry))
> goto out;
>
> @@ -4045,6 +4049,8 @@ static int vfs_rename_other(struct inode *old_dir, struct dentry *old_dentry,
> lock_two_nondirectories(source, target);
>
> error = -EBUSY;
> + if (is_local_mountpoint(old_dentry) || is_local_mountpoint(new_dentry))
> + goto out;
> if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
> goto out;
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 22e536705c45..6dc23614d44d 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -631,6 +631,41 @@ struct vfsmount *lookup_mnt(struct path *path)
> return m;
> }
>
> +/*
> + * __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.
> + */
> +int __is_local_mountpoint(struct dentry *dentry)

Minor nit: return value of any is_* function is either true or false, so why not
declare it bool?

> +{
> + struct mnt_namespace *ns = current->nsproxy->mnt_ns;
> + struct mount *mnt;
> + int is_covered = 0;

And the same for this var.

> +
> + 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);
> --
> 1.7.5.4
>


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