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Mitigate a side-channel leak of which relays Tor chooses for a circuit

Browse Source 
Tor's and OpenSSL's current design guarantee that there are other leaks,
but this one is likely to be more easily exploitable, and is easy to fix.
This commit is contained in:
Robert Ransom 2012-06-14 17:15:54 +00:00 committed by Nick Mathewson
parent 55f635745a
commit 308f6dad20
2 changed files with 30 additions and 4 deletions
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12
changes/pathsel-BUGGY-a Normal file
View File

@ -0,0 +1,12 @@
o Security fixes:
- Try to leak less information about what relays a client is
choosing to a side-channel attacker. Previously, a Tor client
would stop iterating through the list of available relays as
soon as it had chosen one, thus leaking information about which
relays it picked for a circuit to a timing attack. (Tor is
likely to still leak information about which relays it has
chosen for a circuit to other processes on the same computer,
through e.g. which cache lines it loads while building the
circuit.)

22
src/or/routerlist.c
View File

@ -1674,6 +1674,8 @@ smartlist_choose_by_bandwidth_weights(smartlist_t *sl,
double *bandwidths; double *bandwidths;
double tmp = 0; double tmp = 0;
unsigned int i; unsigned int i;
unsigned int i_chosen;
unsigned int i_has_been_chosen;
int have_unknown = 0; /* true iff sl contains element not in consensus. */ int have_unknown = 0; /* true iff sl contains element not in consensus. */
/* Can't choose exit and guard at same time */ /* Can't choose exit and guard at same time */
@ -1835,12 +1837,17 @@ smartlist_choose_by_bandwidth_weights(smartlist_t *sl,
* from 1 below. See bug 1203 for details. */ * from 1 below. See bug 1203 for details. */
/* Last, count through sl until we get to the element we picked */ /* Last, count through sl until we get to the element we picked */
i_chosen = (unsigned)smartlist_len(sl);
i_has_been_chosen = 0;
tmp = 0.0; tmp = 0.0;
for (i=0; i < (unsigned)smartlist_len(sl); i++) { for (i=0; i < (unsigned)smartlist_len(sl); i++) {
tmp += bandwidths[i]; tmp += bandwidths[i];
if (tmp >= rand_bw) if (tmp >= rand_bw && !i_has_been_chosen) {
break; i_chosen = i;
i_has_been_chosen = 1;
}
} }
i = i_chosen;
if (i == (unsigned)smartlist_len(sl)) { if (i == (unsigned)smartlist_len(sl)) {
/* This was once possible due to round-off error, but shouldn't be able /* This was once possible due to round-off error, but shouldn't be able
@ -1877,6 +1884,8 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, bandwidth_weight_rule_t rule,
int statuses) int statuses)
{ {
unsigned int i; unsigned int i;
unsigned int i_chosen;
unsigned int i_has_been_chosen;
routerinfo_t *router; routerinfo_t *router;
routerstatus_t *status=NULL; routerstatus_t *status=NULL;
int32_t *bandwidths; int32_t *bandwidths;
@ -2092,6 +2101,8 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, bandwidth_weight_rule_t rule,
/* Last, count through sl until we get to the element we picked */ /* Last, count through sl until we get to the element we picked */
tmp = 0; tmp = 0;
i_chosen = (unsigned)smartlist_len(sl);
i_has_been_chosen = 0;
for (i=0; i < (unsigned)smartlist_len(sl); i++) { for (i=0; i < (unsigned)smartlist_len(sl); i++) {
is_exit = bitarray_is_set(exit_bits, i); is_exit = bitarray_is_set(exit_bits, i);
is_guard = bitarray_is_set(guard_bits, i); is_guard = bitarray_is_set(guard_bits, i);
@ -2106,9 +2117,12 @@ smartlist_choose_by_bandwidth(smartlist_t *sl, bandwidth_weight_rule_t rule,
else else
tmp += bandwidths[i]; tmp += bandwidths[i];
if (tmp >= rand_bw) if (tmp >= rand_bw && !i_has_been_chosen) {
break; i_chosen = i;
i_has_been_chosen = 1;
}
} }
i = i_chosen;
if (i == (unsigned)smartlist_len(sl)) { if (i == (unsigned)smartlist_len(sl)) {
/* This was once possible due to round-off error, but shouldn't be able /* This was once possible due to round-off error, but shouldn't be able
* to occur any longer. */ * to occur any longer. */