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Remove remaining timing-dependency in choosing nodes by bandwidth

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The old approach, because of its "tmp >= rand_bw &&
!i_has_been_chosen" check, would run through the second part of the
loop slightly slower than the first part.  Now, we remove
i_has_been_chosen, and instead set rand_bw = UINT64_MAX, so that
every instance of the loop will do exactly the same amount of work
regardless of the initial value of rand_bw.

Fix for bug 6538.
This commit is contained in:
Nick Mathewson 2012-08-09 12:37:40 -04:00
parent e106812a77
commit 640a51684c
2 changed files with 12 additions and 8 deletions
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8
changes/bug6538
View File

@ -2,3 +2,11 @@
- Switch weighted node selection rule from using a list of doubles
to using a list of int64_t. This should make the process slightly
easier to debug and maintain. Needed for fix for bug 6538.
o Security features:
- Switch to a completely time-invariant approach for picking nodes
weighted by bandwidth. Our old approach would run through the
part of the loop after it had made its choice slightly slower
than it ran through the part of the loop before it had made its
choice. Fix for bug 6538.

12
src/or/routerlist.c
View File

@ -1710,7 +1710,6 @@ smartlist_choose_node_by_bandwidth_weights(smartlist_t *sl,
uint64_t tmp;
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 */
@ -1881,13 +1880,12 @@ smartlist_choose_node_by_bandwidth_weights(smartlist_t *sl,
/* 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;
for (i=0; i < (unsigned)smartlist_len(sl); i++) {
tmp += bandwidths[i];
if (tmp >= rand_bw && !i_has_been_chosen) {
if (tmp >= rand_bw) {
i_chosen = i;
i_has_been_chosen = 1;
rand_bw = UINT64_MAX;
}
}
i = i_chosen;
@ -1926,7 +1924,6 @@ smartlist_choose_node_by_bandwidth(smartlist_t *sl,
{
unsigned int i;
unsigned int i_chosen;
unsigned int i_has_been_chosen;
int32_t *bandwidths;
int is_exit;
int is_guard;
@ -2127,7 +2124,6 @@ smartlist_choose_node_by_bandwidth(smartlist_t *sl,
/* 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);
@ -2142,9 +2138,9 @@ smartlist_choose_node_by_bandwidth(smartlist_t *sl,
else
tmp += bandwidths[i];
if (tmp >= rand_bw && !i_has_been_chosen) {
if (tmp >= rand_bw) {
i_chosen = i;
i_has_been_chosen = 1;
rand_bw = UINT64_MAX;
}
}
i = i_chosen;