http-walker: fix buffer underflow processing remote alternates

If we parse a remote alternates (or http-alternates), we
expect relative lines like:

../../foo.git/objects

which we convert into "$URL/../foo.git/" (and then use that
as a base for fetching more objects).

But if the remote feeds us nonsense like just:

../

we will try to blindly strip the last 7 characters, assuming
they contain the string "objects". Since we don't _have_ 7
characters at all, this results in feeding a small negative
value to strbuf_add(), which converts it to a size_t,
resulting in a big positive value. This should consistently
fail (since we can't generall allocate the max size_t minus
7 bytes), so there shouldn't be any security implications.

Let's fix this by using strbuf_strip_suffix() to drop the
characters we want. If they're not present, we'll ignore the
alternate (in theory we could use it as-is, but the rest of
the http-walker code unconditionally tacks "objects/" back
on, so it is it not prepared to handle such a case).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/http-walker.c b/http-walker.c
index b34b6ace7cd80a482eae16e2063aa9d8960e8729..507c200f004a0d1e295a6ffe876667ddf20db3bf 100644 (file)
--- a/http-walker.c
+++ b/http-walker.c
@@ -296,13 +296,16 @@ static void process_alternates_response(void *callback_data)
                                        okay = 1;
                                }
                        }
-                       /* skip "objects\n" at end */
                        if (okay) {
                                struct strbuf target = STRBUF_INIT;
                                strbuf_add(&target, base, serverlen);
-                               strbuf_add(&target, data + i, posn - i - 7);
-
-                               if (is_alternate_allowed(target.buf)) {
+                               strbuf_add(&target, data + i, posn - i);
+                               if (!strbuf_strip_suffix(&target, "objects")) {
+                                       warning("ignoring alternate that does"
+                                               " not end in 'objects': %s",
+                                               target.buf);
+                                       strbuf_release(&target);
+                               } else if (is_alternate_allowed(target.buf)) {
                                        warning("adding alternate object store: %s",
                                                target.buf);
                                        newalt = xmalloc(sizeof(*newalt));