curl: check data size before memcpy to local buffer. (CVE-2014-0144)

curl_read_cb is callback function for libcurl when data arrives. The
data size passed in here is not guaranteed to be within the range of
request we submitted, so we may overflow the guest IO buffer. Check the
real size we have before memcpy to buffer to avoid overflow.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 6d4b9e55fc)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

block/curl.c

@@ -157,6 +157,11 @@ static size_t curl_read_cb(void *ptr, size_t size, size_t nmemb, void *opaque)
     if (!s || !s->orig_buf)
         goto read_end;
 
+    if (s->buf_off >= s->buf_len) {
+        /* buffer full, read nothing */
+        return 0;
+    }
+    realsize = MIN(realsize, s->buf_len - s->buf_off);
     memcpy(s->orig_buf + s->buf_off, ptr, realsize);
     s->buf_off += realsize;