On Sun, Mar 26, 2017 at 04:01:28PM +0000, brian m. carlson wrote:
> If we had already processed the last newline in a push certificate, we
> would end up subtracting NULL from the end-of-certificate pointer when
> computing the length of the line. This would have resulted in an
> absurdly large length, and possibly a buffer overflow. Instead,
> subtract the beginning-of-certificate pointer from the
> end-of-certificate pointer, which is what's expected.
>
> Note that this situation should never occur, since not only do we
> require the certificate to be newline terminated, but the signature will
> only be read from the beginning of a line. Nevertheless, it seems
> prudent to correct it.
I think you can trigger it with carefully-crafted input. Try this on the
client side:
diff --git a/send-pack.c b/send-pack.c
index 66e652f7e..dd18c9a33 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -311,8 +311,7 @@ static int generate_push_cert(struct strbuf *req_buf,
if (!update_seen)
goto free_return;
- if (sign_buffer(&cert, &cert, signing_key))
- die(_("failed to sign the push certificate"));
+ strbuf_rtrim(&cert);
packet_buf_write(req_buf, "push-cert%c%s", 0, cap_string);
for (cp = cert.buf; cp < cert.buf + cert.len; cp = np) {
We omit the signature entirely, which causes the parser to treat the
end of the string as the end-of-cert (we still find the end because the
push-cert-end is in its own pkt-line; you could also just issue a flush,
which has the same effect).
And then the rtrim means that the cert doesn't actually end in a
newline. Running t5534 with this patch causes receive-pack to segfault.
It's not an overflow on writing the buffer, though; the nonsense size is
passed into a FLEX_ALLOC_MEM(). In my test case, I ended up allocating a
1.4GB buffer (which just happened to be the mod-2^32 residue of my "eoc"
pointer), and the segfault comes from trying to read an unallocated
page.
So I don't think it's exploitable in any interesting way.
> diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
> index feafb076a4..116f3177a1 100644
> --- a/builtin/receive-pack.c
> +++ b/builtin/receive-pack.c
> @@ -1524,7 +1524,7 @@ static void queue_commands_from_cert(struct command **tail,
>
> while (boc < eoc) {
> const char *eol = memchr(boc, '\n', eoc - boc);
> - tail = queue_command(tail, boc, eol ? eol - boc : eoc - eol);
> + tail = queue_command(tail, boc, eol ? eol - boc : eoc - boc);
> boc = eol ? eol + 1 : eoc;
> }
> }
The patch itself is obviously an improvement. It may be worth graduating
separately from the rest of the series.
-Peff