extcap: fix use-after-free for preferences

In commit v2.3.0rc0-117-g485bc45 (backported to v2.2.0rc0-44-g66721ca),
extcap_prefs_dynamic_vals and extcap_cleanup were added in an attempt to
address dangling pointers.

Unfortunately it is not sufficient:

 - A pointer to the preference value is stored in extcap_arg and passed
   to the prefs API, but this extcap_arg structure can become invalid
   which result in use-after-free whenever the preference is accessed.
 - On exit, a use-after-free occurs in prefs_cleanup when the preference
   value is being checked.

As the preference subsystem actually manages the memory for the string
value and consumers should only provide a pointer where the value can be
stored, convert the char* field in extcap to char**. This has as
additional benefit that values are not limited to 256 bytes anymore.

extcap_cleanup is moved after epan_cleanup to ensure that prefs_cleanup
does not operate on dangling pointers.

Crash is reproducible under ASAN with: tshark -i randpkt

Ping-Bug: 12183
Change-Id: Ibf1ba1102a5633aa085dc278a12ffc05a4f4a34b
Reviewed-on: https://code.wireshark.org/review/17631
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Roland Knall <rknall@gmail.com>

1 files changed, 4 insertions, 12 deletions

diff --git a/rawshark.c b/rawshark.c
index fe68b76f92..510ed80bcf 100644
--- a/rawshark.c
+++ b/rawshark.c
@@ -802,10 +802,8 @@ main(int argc, char *argv[])
                 cmdarg_err("%s", err_msg);
                 g_free(err_msg);
                 epan_free(cfile.epan);
-#ifdef HAVE_EXTCAP
-                extcap_cleanup();
-#endif
                 epan_cleanup();
+                extcap_cleanup();
                 exit(2);
             }
             n_rfcodes++;
@@ -826,10 +824,8 @@ main(int argc, char *argv[])
 
         if (raw_cf_open(&cfile, pipe_name) != CF_OK) {
             epan_free(cfile.epan);
-#ifdef HAVE_EXTCAP
-            extcap_cleanup();
-#endif
             epan_cleanup();
+            extcap_cleanup();
             exit(2);
         }
 
@@ -850,10 +846,8 @@ main(int argc, char *argv[])
         /* Process the packets in the file */
         if (!load_cap_file(&cfile)) {
             epan_free(cfile.epan);
-#ifdef HAVE_EXTCAP
-            extcap_cleanup();
-#endif
             epan_cleanup();
+            extcap_cleanup();
             exit(2);
         }
     } else {
@@ -863,10 +857,8 @@ main(int argc, char *argv[])
     }
 
     epan_free(cfile.epan);
-#ifdef HAVE_EXTCAP
-    extcap_cleanup();
-#endif
     epan_cleanup();
+    extcap_cleanup();
 
     return 0;
 }