Re: [PATCH bpf-next v9 05/10] bpf,landlock: Add a new map type: inode

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jun 25, 2019 at 11:52:34PM +0200, Mickaël Salaün wrote:
> +/* must call iput(inode) after this call */
> +static struct inode *inode_from_fd(int ufd, bool check_access)
> +{
> +	struct inode *ret;
> +	struct fd f;
> +	int deny;
> +
> +	f = fdget(ufd);
> +	if (unlikely(!f.file || !file_inode(f.file))) {
> +		ret = ERR_PTR(-EBADF);
> +		goto put_fd;
> +	}

Just when does one get a NULL file_inode()?  The reason I'm asking is
that arseloads of code would break if one managed to create such
a beast...

Incidentally, that should be return ERR_PTR(-EBADF); fdput() is wrong there.

> +	}
> +	/* check if the FD is tied to a mount point */
> +	/* TODO: add this check when called from an eBPF program too */
> +	if (unlikely(!f.file->f_path.mnt

Again, the same question - when the hell can that happen?  If you are
sitting on an exploitable roothole, do share it...

 || f.file->f_path.mnt->mnt_flags &
> +				MNT_INTERNAL)) {
> +		ret = ERR_PTR(-EINVAL);
> +		goto put_fd;

What does it have to do with mountpoints, anyway?

> +/* called from syscall */
> +static int sys_inode_map_delete_elem(struct bpf_map *map, struct inode *key)
> +{
> +	struct inode_array *array = container_of(map, struct inode_array, map);
> +	struct inode *inode;
> +	int i;
> +
> +	WARN_ON_ONCE(!rcu_read_lock_held());
> +	for (i = 0; i < array->map.max_entries; i++) {
> +		if (array->elems[i].inode == key) {
> +			inode = xchg(&array->elems[i].inode, NULL);
> +			array->nb_entries--;

Umm...  Is that intended to be atomic in any sense?

> +			iput(inode);
> +			return 0;
> +		}
> +	}
> +	return -ENOENT;
> +}
> +
> +/* called from syscall */
> +int bpf_inode_map_delete_elem(struct bpf_map *map, int *key)
> +{
> +	struct inode *inode;
> +	int err;
> +
> +	inode = inode_from_fd(*key, false);
> +	if (IS_ERR(inode))
> +		return PTR_ERR(inode);
> +	err = sys_inode_map_delete_elem(map, inode);
> +	iput(inode);
> +	return err;
> +}

Wait a sec...  So we have those beasties that can have long-term
references to arbitrary inodes stuck in them?  What will happen
if you get umount(2) called while such a thing exists?