Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY_PATH)

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

On Thu, Aug 22, 2013 at 01:54:15PM -0700, Linus Torvalds wrote:
> On Thu, Aug 22, 2013 at 1:48 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > Sure.  But aren't they always last?
> 
> What do you mean? I'd say that the /proc lookup is always *innermost*.
> Which means that it certainly cannot bail out, since there are many
> levels of nesting outside of it.
> 
> > With the current code structure, trying to enforce some kind of
> > security restriction in the middle of lookup seems really unpleasant.
> 
> If it's conditional (ie "linkat behaves differently from openat"), it
> certainly means that we'd have to pass in that info in annoying ways.

Nope.  All we need to pass is one more LOOKUP_...  Add
	if (unlikely(nd->last_type == LAST_BIND)) {
		if ((nd->flags & LOOKUP_BLAH) && !may_flink(...)) {
			terminate_walk(nd);
			return -EINVAL;
		}
	}
in the beginning of lookup_last() and pass LOOKUP_BLAH in flags when
linkat() calls user_path_at().  That will affect *only* the terminal
symlinks and cost nothing in all normal cases.  The same check can
bloody well go into path_init() - take
                if (*name) {
                        if (!can_lookup(dentry->d_inode)) {
                                fdput(f);
                                return -ENOTDIR;
                        }
                }
in there and slap
		else {
			if ((flags & LOOKUP_BLAH) && !may_flink(...)) {
                                fdput(f);
				return -EINVAL;
			}
		}
after it.