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

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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 tmp = 0;
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. */
/* 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. */
/* 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;
for (i=0; i < (unsigned)smartlist_len(sl); i++) {
tmp += bandwidths[i];
if (tmp >= rand_bw)
break;
if (tmp >= rand_bw && !i_has_been_chosen) {
i_chosen = i;
i_has_been_chosen = 1;
}
}
i = i_chosen;
if (i == (unsigned)smartlist_len(sl)) {
/* 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)
{
unsigned int i;
unsigned int i_chosen;
unsigned int i_has_been_chosen;
routerinfo_t *router;
routerstatus_t *status=NULL;
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 */
tmp = 0;
i_chosen = (unsigned)smartlist_len(sl);
i_has_been_chosen = 0;
for (i=0; i < (unsigned)smartlist_len(sl); i++) {
is_exit = bitarray_is_set(exit_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
tmp += bandwidths[i];
if (tmp >= rand_bw)
break;
if (tmp >= rand_bw && !i_has_been_chosen) {
i_chosen = i;
i_has_been_chosen = 1;
}
}
i = i_chosen;
if (i == (unsigned)smartlist_len(sl)) {
/* This was once possible due to round-off error, but shouldn't be able
* to occur any longer. */