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Require a threshold of exit nodes before building circuits

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This mitigates an attack proposed by wanoskarnet, in which all of a
client's bridges collude to restrict the exit nodes that the client
knows about.  Fixes bug 5343.
This commit is contained in:
Nick Mathewson 2012-03-08 15:42:54 -05:00
parent fe50b676bc
commit ec8a06c5a1
2 changed files with 31 additions and 5 deletions
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7
changes/bug5343 Normal file
View File

@ -0,0 +1,7 @@
o Security fixes:
- Only build circuits if we have a sufficient threshold of the total
descriptors marked in the consensus with the "Exit" flag. This
mitigates an attack proposed by wanoskarnet, in which all of a
client's bridges collude to restrict the exit nodes that the
client knows about. Fixes bug 5343.

29
src/or/routerlist.c
View File

@ -4788,18 +4788,21 @@ get_dir_info_status_string(void)
* them seem like ones we'd use, and how many of <em>those</em> we have
* descriptors for. Store the former in *<b>num_usable</b> and the latter in
* *<b>num_present</b>. If <b>in_set</b> is non-NULL, only consider those
* routers in <b>in_set</b>.
* routers in <b>in_set</b>. If <b>exit_only</b> is true, only consider nodes
* with the Exit flag.
*/
static void
count_usable_descriptors(int *num_present, int *num_usable,
const networkstatus_t *consensus,
or_options_t *options, time_t now,
routerset_t *in_set)
routerset_t *in_set, int exit_only)
{
*num_present = 0, *num_usable=0;
SMARTLIST_FOREACH(consensus->routerstatus_list, routerstatus_t *, rs,
{
if (exit_only && ! rs->is_exit)
continue;
if (in_set && ! routerset_contains_routerstatus(in_set, rs))
continue;
if (client_would_use_router(rs, now, options)) {
@ -4830,7 +4833,7 @@ count_loading_descriptors_progress(void)
return 0; /* can't count descriptors if we have no list of them */
count_usable_descriptors(&num_present, &num_usable,
consensus, get_options(), now, NULL);
consensus, get_options(), now, NULL, 0);
if (num_usable == 0)
return 0; /* don't div by 0 */
@ -4849,6 +4852,7 @@ static void
update_router_have_minimum_dir_info(void)
{
int num_present = 0, num_usable=0;
int num_exit_present = 0, num_exit_usable = 0;
time_t now = time(NULL);
int res;
or_options_t *options = get_options();
@ -4875,7 +4879,9 @@ update_router_have_minimum_dir_info(void)
}
count_usable_descriptors(&num_present, &num_usable, consensus, options, now,
NULL);
NULL, 0);
count_usable_descriptors(&num_exit_present, &num_exit_usable,
consensus, options, now, options->ExitNodes, 1);
if (num_present < num_usable/4) {
tor_snprintf(dir_info_status, sizeof(dir_info_status),
@ -4889,12 +4895,25 @@ update_router_have_minimum_dir_info(void)
num_present, num_present ? "" : "s");
res = 0;
goto done;
} else if (num_exit_present < num_exit_usable / 3) {
tor_snprintf(dir_info_status, sizeof(dir_info_status),
"We have only %d/%d usable exit node descriptors.",
num_exit_present, num_exit_usable);
res = 0;
control_event_bootstrap(BOOTSTRAP_STATUS_REQUESTING_DESCRIPTORS, 0);
goto done;
} else if (num_exit_present < 2) {
tor_snprintf(dir_info_status, sizeof(dir_info_status),
"Only %d descriptor%s here and believed reachable!",
num_exit_present, num_exit_present ? "" : "s");
res = 0;
goto done;
}
/* Check for entry nodes. */
if (options->EntryNodes) {
count_usable_descriptors(&num_present, &num_usable, consensus, options,
now, options->EntryNodes);
now, options->EntryNodes, 0);
if (!num_usable || !num_present) {
tor_snprintf(dir_info_status, sizeof(dir_info_status),