From 3dd905eaa258aa3d11640dfce2135e4af82a620f Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Fri, 4 Oct 2013 19:15:13 -0700 Subject: [PATCH] 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 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 Signed-off-by: "Eric W. Biederman" --- fs/mount.h | 9 +++++++++ fs/namei.c | 8 +++++++- fs/namespace.c | 35 +++++++++++++++++++++++++++++++++++ 3 files changed, 51 insertions(+), 1 deletion(-) diff --git a/fs/mount.h b/fs/mount.h index a17458ca6f29..18511835dc28 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 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 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..abe2f56efaea 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. + */ +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); -- 2.39.5