Re: netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

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

On Fri, Dec 01, 2017 at 01:33:04AM +0000, Al Viro wrote:

> Use of file descriptors should be limited to "got a number from userland,
> convert to struct file *" on the way in and "install struct file * into
> descriptor table and return the descriptor to userland" on the way out.
> And the latter - *ONLY* after the last possible point of failure.  Once
> a file reference is inserted into descriptor table, that's it - you
> can't undo that.
> 
> The only way to use bpf_obj_get_user() is to pass its return value to
> userland.  As return value of syscall - not even put_user() (for that
> you'd need to reserve the descriptor, copy it to userland and only
> then attach struct file * to it).
> 
> The whole approach stinks - what it needs is something that would
> take struct filename * and return struct bpf_prog * or struct file *
> reference.  With bpf_obj_get_user() and this thing implemented
> via that.
> 
> I'm looking into that thing...

What it tries to pull off is something not far from

static struct bpf_prog *__get_prog(struct inode *inode, enum bpf_prog_type type)
{
	struct bpf_prog *prog;
	int err = inode_permission(inode, FMODE_READ | FMODE_WRITE);
	if (err)
		return ERR_PTR(err);

	if (inode->i_op == &bpf_map_iops)
		return ERR_PTR(-EINVAL);

	if (inode->i_op != &bpf_prog_iops)
		return ERR_PTR(-EACCES);

	prog = inode->i_private;
	err = security_bpf_prog(prog);
	if (err < 0)
		return ERR_PTR(err);

	if (!bpf_prog_get_ok(prog, &type, false))
		return ERR_PTR(-EINVAL);

	return bpf_prog_inc(prog);
}

struct bpf_prog *get_prog_path_type(const char *name, enum bpf_prog_type type)
{
	struct path path;
	struct bpf_prog *prog;
	int err = kern_path(name, LOOKUP_FOLLOW, &path);
	if (err)
		return ERR_PTR(err);
	prog = __get_prog(d_backing_inode(path.dentry), type);
	if (!IS_ERR(prog))
		touch_atime(&path);
	path_put(&path);
	return prog;
}

static int __bpf_mt_check_path(const char *path, struct bpf_prog **ret)
{
	*ret = get_prog_path_type(path, BPF_PROG_TYPE_SOCKET_FILTER);
        return PTR_ERR_OR_ZERO(*ret);
}

That skips all tracepoint random shite (pardon the triple redundance) and makes
a somewhat arbitrary change for touch_atime() logics.  And, of course, it is
not even compile-tested.

Something similar to get_prog_path_type() above might make for a usable
primitive, IMO...