On 08/09, Jeff King wrote: > On Wed, Aug 08, 2018 at 03:33:23PM -0700, Brandon Williams wrote: > > > Commit 0383bbb901 (submodule-config: verify submodule names as paths, > > 2018-04-30) introduced some checks to ensure that submodule names don't > > include directory traversal components (e.g. "../"). > > > > This addresses the vulnerability identified in 0383bbb901 but the root > > cause is that we use submodule names to construct paths to the > > submodule's git directory. What we really should do is munge the > > submodule name before using it to construct a path. > > > > Teach "submodule_name_to_gitdir()" to munge a submodule's name (by url > > encoding it) before using it to build a path to the submodule's gitdir. > > I like this approach very much, and I think using url encoding is much > better than an opaque hash (purely because it makes debugging and > inspection saner). > > Two thoughts, though: > > > + modules_len = buf->len; > > strbuf_addstr(buf, submodule_name); > > + > > + /* > > + * If the submodule gitdir already exists using the old-fashioned > > + * location (which uses the submodule name as-is, without munging it) > > + * then return that. > > + */ > > + if (!access(buf->buf, F_OK)) > > + return; > > 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!". > > In theory a fresh clone could set a config option for "I only speak > use new-style modules". And there could even be a conversion program > that moves the modules as appropriate, fixes up the .git files in the > working tree, and then sets that config. > > In fact, I think that config option _could_ be done by bumping > core.repositoryformatversion and then setting extensions.submodulenames > to "url" or something. Then you could never run into the confusing case > where you have a clone done by a new version of git (using new-style > names), but using an old-style version gets confused because it can't > find the module directories (instead, it would barf and say "I don't > know about that extension"). > > I don't know if any of that is worth it, though. We already fixed the > problem from 0383bbb901. There may be a _different_ "break out of the > modules directory" vulnerability, but since we disallow ".." it's hard > to see what it would be (the best I could come up with is maybe pointing > one module into the interior of another module, but I think you'd have > to trouble overwriting anything useful). > > And while an old-style version of Git being confused might be annoying, > I suspect that bumping the repository version would be even _more_ > annoying, because it would hit every command, not just ones that try to > touch those submodules. 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. > > > diff --git a/t/t7400-submodule-basic.sh b/t/t7400-submodule-basic.sh > > index 2c2c97e144..963693332c 100755 > > --- a/t/t7400-submodule-basic.sh > > +++ b/t/t7400-submodule-basic.sh > > @@ -933,7 +933,7 @@ test_expect_success 'recursive relative submodules stay relative' ' > > cd clone2 && > > git submodule update --init --recursive && > > echo "gitdir: ../.git/modules/sub3" >./sub3/.git_expect && > > - echo "gitdir: ../../../.git/modules/sub3/modules/dirdir/subsub" >./sub3/dirdir/subsub/.git_expect > > + echo "gitdir: ../../../.git/modules/sub3/modules/dirdir%2fsubsub" >./sub3/dirdir/subsub/.git_expect > > 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. > > Some possible actions: > > 0. Do nothing, and cross our fingers. ;) > > 1. Don't use strbuf_addstr_urlencode(), but rather our own munging > function which we know will remain stable (or alternatively, a flag > to strbuf_addstr_urlencode to get the consistent behavior). > > 2. Make sure we have tests which cover this, so at least somebody > changing the urlencode decisions will see a breakage. Your test here > covers the upper/lowercase one, but we might want one that covers > "+". (There may be more ambiguous cases, but those are the ones I > know about). > > 3. Rather than check for the existence of names, decode what's actually > in the modules/ directory to create an in-memory index of names. > > I hesitate to suggest that, because it's obviously way more > complicated, and may perform worse if you have a lot of modules > (since you have to readdir() and decode the whole directory just to > look up one module). > > But I think it also gives a more elegant solution to the > backwards-compatibility problem, since we could recognize both new > and old-style names. There's some ambiguity (e.g., is "foo%2fbar" > "foo/bar", or did somebody really have a name with a percent in > it?),. but in theory you could respect either name (giving > preference to new-style in case of a conflict). > > And I think the result would be immune to any directory-escape > vulnerabilities, because we'd always start with what actually exists > in $GIT_DIR/modules/, which we know _we_ will have written. > > Again, I'm not sure if it's worth the effort, but I thought I'd > throw it out there. > > -Peff -- Brandon Williams
