[tor-commits] [tor/maint-0.4.1] Fix a crash bug in max_u16_in_sl()

[nickm at torproject.org]

commit 2b825a1a2e6e79fa71b0e038241d2107aaf30d4b
Author: Nick Mathewson <nickm at torproject.org>
Date:   Tue Oct 1 12:55:42 2019 -0400

    Fix a crash bug in max_u16_in_sl()
    
    The documentation for this function says that the smartlist can
    contain NULLs, but the code only handled NULLs if they were at the
    start of the list.
    
    We didn't notice this for a long time, because when Tor is run
    normally, the sequence of msg_id_t is densely packed, and so this
    list (mapping msg_id_t to channel_id_t) contains no NULL elements.
    We could only run into this bug:
      * when Tor was running in embedded mode, and starting more than once.
      * when Tor ran first with more pubsub messages enabled, and then
        later with fewer.
      * When the second run (the one with fewer enabled pubsub messages)
        had at least some messages enabled, and those messages were not
        the ones with numerically highest msg_id_t values.
    
    Fixes bug 31898; bugfix on 47de9c7b0a828de7fb8129413db70bc4e4ecac6d
    in 0.4.1.1-alpha.
---
 changes/bug31898                | 4 ++++
 src/lib/dispatch/dispatch_new.c | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/changes/bug31898 b/changes/bug31898
new file mode 100644
index 000000000..6f3e0a546
--- /dev/null
+++ b/changes/bug31898
@@ -0,0 +1,4 @@
+  o Major bugfixes (embedded Tor):
+    - Avoid a possible crash when restarting Tor in embedded mode and
+      enabling a different set of publish/subscribe messages. Fixes bug
+      31898; bugfix on 0.4.1.1-alpha.
diff --git a/src/lib/dispatch/dispatch_new.c b/src/lib/dispatch/dispatch_new.c
index 446781306..d8e59d610 100644
--- a/src/lib/dispatch/dispatch_new.c
+++ b/src/lib/dispatch/dispatch_new.c
@@ -34,7 +34,7 @@ max_in_u16_sl(const smartlist_t *sl, int dflt)
   SMARTLIST_FOREACH_BEGIN(sl, uint16_t *, u) {
     if (!maxptr)
       maxptr = u;
-    else if (*u > *maxptr)
+    else if (u && *u > *maxptr)
       maxptr = u;
   } SMARTLIST_FOREACH_END(u);