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[PATCH] http-walker: fix buffer underflow processing remote alternates

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


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 generally 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. As a bonus this lets us handle names
that do not end in "objects" (all git repos do, but there is
nothing to say that an alternate object store needs to be a
git repo).

And while we're here, we can add a few other parsing
niceties, like dropping trailing whitespace, and handling
names that do not end in "/".

Signed-off-by: Jeff King <peff@xxxxxxxx>
I posted this last week in the middle of another thread[1], but it
didn't get any attention. So here it is again.

I admit I don't care at all about http-alternates, but potential
out-of-range errors worry me.

 http-walker.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/http-walker.c b/http-walker.c
index b34b6ace7..d62ca8953 100644
--- a/http-walker.c
+++ b/http-walker.c
@@ -296,11 +296,13 @@ 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);
+				strbuf_add(&target, data + i, posn - i);
+				strbuf_rtrim(&target);
+				strbuf_strip_suffix(&target, "objects");
+				strbuf_complete(&target, '/');
 				if (is_alternate_allowed(target.buf)) {
 					warning("adding alternate object store: %s",