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Message-ID: <20160316001114.GC4884@boyd>
Date: Tue, 15 Mar 2016 19:11:15 -0500
From: Tyler Hicks <tyhicks@...onical.com>
To: oss-security@...ts.openwall.com
Subject: Re: server and client side remote code execution through a buffer overflow in all git versions before 2.7.1 (unpublished ᴄᴠᴇ-2016-2324 and ᴄᴠᴇ‑2016‑2315)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 - Thank you for reporting these issues to the list.
It seems like there's some confusion about which specific issues
CVE-2016-2315 and CVE-2016-2324 actually correspond to.
Debian's Security Tracker has some links to commits for each issue and,
from that, I've tried to make sense of each:
* CVE-2016-2315 is listed as being fixed by
https://github.com/git/git/commit/34fa79a6cde56d6d428ab0d3160cb094ebad3305
- Suggesting to me that CVE-2016-2315 is the more serious buffer
overflow issue.
* CVE-2016-2324 is listed as being fixed by
https://github.com/git/git/commit/9831e92bfa833ee9c0ce464bbc2f941ae6c2698d
- Does this simply fix the issue of allocating much more memory than
needed?
Is this correct? Thanks!
Tyler
On 2016-03-15 15:55:37, Laël Cellier wrote:
> Hello, original report describing the overflow is here
> http://pastebin.com/UX2P2jjg
>
>
> 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() ). while it
> seems normal the ᴄᴠᴇ details are still unpublished, I definitely can’t deal
> with every major provider.
>
> People surely remember https://www.google.fr/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.
> So some Peoples suggested me to post about it here.
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