oss-sec mailing list archives

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