Date: Wed, 16 Mar 2016 01:31:47 +0100
From: Laël Cellier <lael.cellier@...oste.net>
To: bugtraq@...urityfocus.com, fulldisclosure@...lists.org,
 security-basics@...urityfocus.com, full-disclosure@...ts.grok.org.uk
Subject: [FD] server and client side remote code execution through a buffer overflow in all git versions before 2.7.1 (unpublished ᴄᴠᴇ-2016-2324 and ᴄᴠᴇ‑2016‑2315)

Hello, original report describing the first overflow full details is 
here http://pastebin.com/UX2P2jjg or at the attachment
The aim is to push a crafted tree object if the target is a server or 
make a client cloning a crafted repository.

On 11/02/2016 16:50, Jeff King wrote this on the git security mailing list:
> On Thu, Feb 11, 2016 at 02:31:49PM +0100, 'Laël Cellier' via Git 
> Security wrote:
>> Ok the bug works by pushing or cloning a repository with a large
>> filename or a large number of nested trees.
>> [...]
>> The point is affected versions are still shipped as part of many
>> distributions as part of their stable branch, so I think it’s
>> important to get a ᴄᴠᴇ for public awareness.
> Yes, I do think versions below v2.7.0 have a heap overflow, as you
> mentioned. But I don't think that is the only problem with path_name(),
> even in the current version.
>
> I'll repeat the code here (the version you posted was indented badly,
> and I had trouble reading it):
>
> -- >8 --
> char *path_name(const struct name_path *path, const char *name)
> {
>          const struct name_path *p;
>          char *n, *m;
>          int nlen = strlen(name);
>          int len = nlen + 1;
>
>          for (p = path; p; p = p->up) {
>                  if (p->elem_len)
>                          len += p->elem_len + 1;
>          }
>          n = xmalloc(len);
>          m = n + len - (nlen + 1);
>          memcpy(m, name, nlen + 1);
>          for (p = path; p; p = p->up) {
>                  if (p->elem_len) {
>                          m -= p->elem_len + 1;
>                          memcpy(m, p->elem, p->elem_len);
>                          m[p->elem_len] = '/';
>                  }
>          }
>          return n;
> }
> -- 8< --
>
> The problem you describe is one where the size of the allocation does
> not match what strcpy would write. And that's kind-of fixed by moving to
> memcpy() in 34fa79a6, because at least now the initial value of "len"
> matches the number of bytes we write (so that number might be totally
> bogus, but we don't write more than we allocate).
>
> But "len" can also change after the fact, due to the loop. If you have a
> sequence of path components, each less than 2^31, they can sum to a much
> smaller positive value due to integer overflow (e.g., A/B/C with lengths
> A=2^31-5, B=2^31-5, C=20 would yield len=10). Then the buffer is too
> small to fit C, let alone all of the extra components we insert in the
> second loop.
>
> The fix I came up with for this is to convert all of the "int" variables
> here to "size_t". That doesn't actually _fix_ the problem at all, but
> does mean on a 64-bit system that you need a 2^64-long path to trigger
> it, which is impractical. But that doesn't help 32-bit systems (though
> in practice, I wouldn't be surprised if we barf long before that, as we
> would be unable to hold the "struct name_path" list in memory).
>
> Note that there is also a similar problem in tree-diff.c's
> path_appendnew().  There we build up the full pathname in a strbuf,
> which checks for overflow. But we then pass that length as an int and
> allocate a FLEX_ARRAY struct with it, which can end up too-small. This
> one is the more interesting of the two, I think, as it triggers via
> git-log, whereas the path_name() happens only during a repack (so it
> will hit you _eventually_, but probably not as soon as you've cloned).
>
> My solution there was similar: use size_t, which at least means you'd
> have to allocate petabytes on a 64-bit system to trigger it (much less
> on a 32-bit system, but _probably_ you'd be saved by malloc failing
> first).
>
> And that's why I dragged my feet on sending those fixes upstream; I
> don't think they're complete. The complete fix would be to use size_t
> consistently to store return values for strlen(), and to do integer
> overflow checks whenever we do computations on size_t.
>
> Those of you on this list may recall I posted a series for the latter
> last year, but it was somewhat invasive. It may be worth resurrecting.
>
> I think we could also get rid of path_name() entirely. The sole purpose
> at this point is to compute the name-hash for pack-objects, which could
> be done by walking the name_path list rather than re-constructing the
> whole thing in memory.
>
> -Peff
Of course everything Peff talked about above is now fixed in git 2.7.1 
with the removal of path_name() and the size_t/overflow check in 
tree-diff.c. It was even fixed earlier for users of github enterprise.
However, several months after the last message on this thread, I’m not 
aware of any Linux distribution that issued a fix for their stable 
branch. Last week I could contact wikimedia so they could fix their 
gerrit‑gc server. Bitbucket, GitLab still suffer from that issue (they 
even use a git version before 
git/commit/34fa79a6cde56d6d428ab0d3160cb094ebad3305 which is the easiest 
one to trigger because of strcpy() instead of memcpy() ).
Users of gerrit are also affected due to gerrit‑gc (so even probably 
google’s servers). But as the frontend is Jgit, the installed git 
version number is hidden (so the only way is to exploit remote code 
execution).
While it seems normal the ᴄᴠᴇ details are still unpublished, I 
definitely can’t deal with every major provider. Their probably many 
popular git services which are still impacted, I just can’t check for 
everyone of them (for example I didn’t checked sourceware.org the site 
that allow to download glibc)

People surely remember 
https://www.google.com/search?tbm=nws&q=CVE-2014-9390 breaking the news 
about a similar issue in that software (which allowed most distros to 
fix it quikcly). It seems while this threat is more widespread, it 
definitely lacks advertisement. I reported it to cert in December but 
didn’t got any response except the automated acknowledgement.
So some peoples suggested me to post about it at security mailing lists.

Those buffer overflows are reported simply as memory corruptions. While 
I’m sure affected variables aren’t at the end of allocated heap, I 
definitely lack the required skills to produce a proof for remote code 
execution http://security.stackexchange.com/q/117394/36301 (I know how 
to put arbitrary data in paths but I don’t know how to exploit a heap 
overflow even without aslr and dep). Being too lazy, I didn’t get the 
required mark to go at the university which could have taught it (I had 
to go at an another one). So I won’t write it.
So if someone is interested in producing such proof … Please do it …


But the primary concern is how to solve that advertisement problem ? Any 
ideas ?

View attachment "cve-2016-2315.txt" of type "text/plain" (1697 bytes)


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