On Thu, May 4, 2017 at 5:30 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > On Mon, May 01, 2017 at 07:36:52PM +0200, Jann Horn wrote: > >> Oh, nice! >> >> It looks like this is somewhat similar to the old O_BENEATH proposal, >> but because the intentions behind the proposals are different >> (application sandboxing versus permitting an application to restrict its >> own filesystem accesses), the semantics differ: AT_NO_JUMPS >> doesn't prevent starting the path with "/", but does prevent mountpoint >> traversal. Is that correct? > > It prevents both, actually - I missed that in description, but this > if (unlikely(nd->flags & LOOKUP_NO_JUMPS)) > return -ELOOP; > in nd_jump_root() affects absolute pathnames same way as it affects > absolute symlinks. > > It's not quite O_BENEATH, and IMO it's saner that way - a/b/c/../d is > bloody well allowed, and so are relative symlinks that do not lead out of > the subtree. If somebody has a good argument in favour of flat-out > ban on .. (_other_ than "other guys do it that way, and it doesn't need > to make sense 'cuz security!!1!!!", please), I'd be glad to hear it. I don't have an argument for allowing '..'. I think it would be okay to disallow it, but I don't think it matters all that much either way. > > As for mountpoint crossing... it might make sense to split those. > O_BENEATH allowed it, and if we want AT_BENEATH to match that - let's > do it. Then this one would become AT_BENEATH | AT_XDEV (the latter named > after find(1) option, obviously). > > So how about this: > > AT_BENEATH: > * no absolute pathnames > * no absolute symlinks > * no procfs-style symlinks > * no traversal of .. when we are at the same place where we'd started > (dir/../file is allowed, dir/../.. isn't) > > AT_XDEV: > * no mountpoint crossing allowed > > For the latter I would prefer -EXDEV, for obvious reasons. For the former... > not sure. I'm not too happy about -ELOOP, but -EPERM (as with O_BENEATH) > is an atrocity - it's even more overloaded. > > Suggestions? -EDOTDOT would be amusing.
