doh: fix (harmless) buffer overrun

Added unit test case 1655 to verify.
Close #4352

the code correctly finds the flaws in the old code,
if one temporarily restores doh.c to the old version.

1 files changed, 110 insertions, 0 deletions

diff --git a/tests/unit/unit1655.c b/tests/unit/unit1655.c
new file mode 100644
index 000000000..60f43d7d6
--- /dev/null
+++ b/tests/unit/unit1655.c
@@ -0,0 +1,110 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+#include "curlcheck.h"
+
+#include "doh.h" /* from the lib dir */
+
+static CURLcode unit_setup(void)
+{
+  /* whatever you want done first */
+  return CURLE_OK;
+}
+
+static void unit_stop(void)
+{
+    /* done before shutting down and exiting */
+}
+
+UNITTEST_START
+
+/* introduce a scope and prove the corner case with write overflow,
+ * so we can prove this test would detect it and that it is properly fixed
+ */
+do {
+const char *bad = "this.is.a.hostname.where.each.individual.part.is.within."
+                  "the.sixtythree.character.limit.but.still.long.enough.to."
+                  "trigger.the.the.buffer.overflow......it.is.chosen.to.be."
+                  "of.a.length.such.that.it.causes.a.two.byte.buffer......."
+                  "overwrite.....making.it.longer.causes.doh.encode.to....."
+                  ".return.early.so.dont.change.its.length.xxxx.xxxxxxxxxxx"
+                  "..xxxxxx.....xx..........xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+                  "xxxxxxxxxxxxxxxxxxxxxxxxxx.xxxxxxxxxxxxxxxx..x......xxxx"
+                  "xxxx..xxxxxxxxxxxxxxxxxxx.x...xxxx.x.x.x...xxxxx";
+
+/* plays the role of struct dnsprobe in urldata.h */
+struct demo {
+    unsigned char dohbuffer[512];
+    unsigned char canary1;
+    unsigned char canary2;
+    unsigned char canary3;
+};
+
+size_t olen = 100000;
+struct demo victim;
+victim.canary1 = 87; /* magic numbers, arbritrarily picked */
+victim.canary2 = 35;
+victim.canary3 = 41;
+DOHcode d = doh_encode(bad, DNS_TYPE_A, victim.dohbuffer,
+                       sizeof(victim.dohbuffer), &olen);
+fail_unless(victim.canary1 == 87, "one byte buffer overwrite has happened");
+fail_unless(victim.canary2 == 35, "two byte buffer overwrite has happened");
+fail_unless(victim.canary3 == 41, "three byte buffer overwrite has happened");
+if(d == DOH_OK)
+{
+  fail_unless(olen <= sizeof(victim.dohbuffer), "wrote outside bounds");
+  fail_unless(olen > strlen(bad), "unrealistic low size");
+}
+} while(0);
+
+/* run normal cases and try to trigger buffer length related errors */
+do {
+DNStype dnstype = DNS_TYPE_A;
+unsigned char buffer[128];
+const size_t buflen = sizeof(buffer);
+const size_t magic1 = 9765;
+size_t olen1 = magic1;
+const char *sunshine1 = "a.com";
+const char *sunshine2 = "aa.com";
+
+DOHcode ret = doh_encode(sunshine1, dnstype, buffer, buflen, &olen1);
+fail_unless(ret == DOH_OK, "sunshine case 1 should pass fine");
+fail_if(olen1 == magic1, "olen has not been assigned properly");
+fail_unless(olen1 > strlen(sunshine1), "bad out length");
+
+/* add one letter, the response should be one longer */
+size_t olen2 = magic1;
+DOHcode ret2 = doh_encode(sunshine2, dnstype, buffer, buflen, &olen2);
+fail_unless(ret2 == DOH_OK, "sunshine case 2 should pass fine");
+fail_if(olen2 == magic1, "olen has not been assigned properly");
+fail_unless(olen1 + 1 == olen2, "olen should grow with the hostname");
+
+/* pass a short buffer, should fail */
+size_t olen;
+ret = doh_encode(sunshine1, dnstype, buffer, olen1 - 1, &olen);
+fail_if(ret == DOH_OK, "short buffer should have been noticed");
+
+/* pass a minimum buffer, should succeed */
+ret = doh_encode(sunshine1, dnstype, buffer, olen1, &olen);
+fail_unless(ret == DOH_OK, "minimal length buffer should be long enough");
+fail_unless(olen == olen1, "bad buffer length");
+} while(0);
+UNITTEST_STOP