Re: [PATCH 2/2] submodule: munge paths to submodule git directories

[Jeff King <peff@peff.net>]

On Tue, Aug 14, 2018 at 11:04:06AM -0700, Brandon Williams wrote:

> > I think this backwards-compatibility is necessary to avoid pain. But
> > until it goes away, I don't think this is helping the vulnerability from
> > 0383bbb901. Because there the issue was that the submodule name pointed
> > back into the working tree, so this access() would find the untrusted
> > working tree code and say "ah, an old-fashioned name!".
> [...]
> 
> Oh I know that this doesn't help with that vulnerability.  As you've
> said we fix it and now disallow ".." at the submodule-config level so
> really this path is simply about using what we get out of
> submodule-config in a more sane manor.

OK, I'm alright with that as long as we are all on the same page. I
think I mistook "this addresses the vulnerability" from your commit
message the wrong way. I took it as "this patch", but reading it again,
you simply mean "the '..' handling we already did".

I do think eventually dropping this back-compatibility could save us
from another directory-escape problem, but it's hard to justify the
real-world pain for a hypothetical benefit. Maybe in a few years we
could get rid of it in a major version bump.

> > One interesting thing about url-encoding is that it's not one-to-one.
> > This case could also be %2F, which is a different file (on a
> > case-sensitive filesystem). I think "%20" and "+" are similarly
> > interchangeable.
> > 
> > If we were decoding the filenames, that's fine. The round-trip is
> > lossless.
> > 
> > But that's not quite how the new code behaves. We encode the input and
> > then check to see if it matches an encoding we previously performed. So
> > if our urlencode routines ever change, this will subtly break.
> > 
> > I don't know how much it's worth caring about. We're not that likely to
> > change the routines ourself (though certainly a third-party
> > implementation would need to know our exact url-encoding decisions).
> 
> This is exactly the reason why I wanted to get some opinions on what the
> best thing to do here would be.  I _think_ the best thing would probably
> be to write a specific routine to do the conversion, and it wouldn't
> even have to be all that complex.  Basically I'm just interested in
> converting '/' characters so that things no longer behave like
> nested directories.

I think we benefit from catching names that would trigger filesystem
case-folding, too. If I have submodules with names "foo" and "FOO", we
would not want to confuse them (or at least we should confuse them
equally on all platforms). I doubt you can do anything malicious, but it
might simply be annoying.

That implies to me using a custom function (even if its encoded form
ends up being understandable as url-encoding).

-Peff