[tor-commits] [tor/maint-0.2.9] Treat an unexpected constant-sized VERSIONS cell as a PROTOCOL_WARN.

[teor at torproject.org]

commit 3c97ab3c24ba4a133377c7ec6ec89cc6903ffb2e
Author: Nick Mathewson <nickm at torproject.org>
Date:   Tue Sep 17 09:09:36 2019 -0400

    Treat an unexpected constant-sized VERSIONS cell as a PROTOCOL_WARN.
    
    We previously used tor_fragile_assert() to declare that this case
    could not happen: VERSIONS cells are always supposed to be
    variable-sized, right?
    
    This is incorrect, though.  On a v1 link protocol connection, all
    cells are fixed-sized.  There aren't supposed to be any VERSIONS
    cells with this version of the protocol, but apparently, somebody
    was messing up.  (The v1 link protocol is obsolete, so probably the
    implementer responsible didn't mean to be using it.)
    
    Fixes bug 31107.  Bugfix on 0.2.4.4-alpha, when we introduced a
    tor_fragile_assert() for this case.
---
 changes/bug31107    |  4 ++++
 src/or/channeltls.c | 10 +++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/changes/bug31107 b/changes/bug31107
new file mode 100644
index 000000000..9652927c3
--- /dev/null
+++ b/changes/bug31107
@@ -0,0 +1,4 @@
+  o Minor bugfixes (logging, protocol violations):
+    - Do not log a nonfatal assertion failure when receiving a VERSIONS
+      cell on a connection using the obsolete v1 link protocol. Log a
+      protocol_warn instead. Fixes bug 31107; bugfix on 0.2.4.4-alpha.
diff --git a/src/or/channeltls.c b/src/or/channeltls.c
index d44f71913..6f4e413dc 100644
--- a/src/or/channeltls.c
+++ b/src/or/channeltls.c
@@ -1098,7 +1098,15 @@ channel_tls_handle_cell(cell_t *cell, or_connection_t *conn)
       /* do nothing */
       break;
     case CELL_VERSIONS:
-      tor_fragile_assert();
+      /* A VERSIONS cell should always be a variable-length cell, and
+       * so should never reach this function (which handles constant-sized
+       * cells). But if the connection is using the (obsolete) v1 link
+       * protocol, all cells will be treated as constant-sized, and so
+       * it's possible we'll reach this code.
+       */
+      log_fn(LOG_PROTOCOL_WARN, LD_CHANNEL,
+             "Received unexpected VERSIONS cell on a channel using link "
+             "protocol %d; ignoring.", conn->link_proto);
       break;
     case CELL_NETINFO:
       ++stats_n_netinfo_cells_processed;