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Bug 25870: Prevent the creation of A - B - A vanguard sub-paths.

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These paths are illegal in Tor and relays will reject them.

We do this by using specific nodes in the exclude list (but ignore /16 and
family).
This commit is contained in:
Mike Perry 2018-05-01 00:59:10 +00:00
parent d634c1ba6b
commit e34bf50604
1 changed files with 57 additions and 24 deletions
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81
src/or/circuitbuild.c
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@ -2444,13 +2444,57 @@ cpath_get_n_hops(crypt_path_t **head_ptr)
#endif /* defined(TOR_UNIT_TESTS) */
/**
* Build the exclude list for vanguard circuits.
*
* For vanguard circuits we exclude all the already chosen nodes (including
* the exit) from being middle hops.
*
* For vanguard circuits, we don't apply any subnet or family restrictions.
* This is to avoid impossible-to-build circuit paths, or just situations where
* our earlier guards prevent us from using most of our later ones.
*
* The alternative is building the circuit in reverse. Reverse calls to
* onion_extend_cpath() (ie: select outer hops first) would then have the
* property that you don't gain information about inner hops by observing
* outer ones. See https://trac.torproject.org/projects/tor/ticket/24487
* for this.
*
* (Note further that we still exclude the exit to prevent A - B - A
* at the end of the path. */
static smartlist_t *
build_vanguard_middle_exclude_list(uint8_t purpose,
cpath_build_state_t *state,
crypt_path_t *head,
int cur_len)
{
smartlist_t *excluded;
const node_t *r;
crypt_path_t *cpath;
int i;
(void) purpose;
excluded = smartlist_new();
/* Add the exit to the exclude list (note that the exit/last hop is always
* chosen first in circuit_establish_circuit()). */
if ((r = build_state_get_exit_node(state))) {
smartlist_add(excluded, (node_t*)r);
}
for (i = 0, cpath = head; cpath && i < cur_len; ++i, cpath=cpath->next) {
if ((r = node_get_by_id(cpath->extend_info->identity_digest))) {
smartlist_add(excluded, (node_t*)r);
}
}
return excluded;
}
/**
* Build a list of nodes to exclude from the choice of this middle
* hop, based on already chosen nodes.
*
* XXX: At present, this function does not exclude any nodes from
* the vanguard circuits. See
* https://trac.torproject.org/projects/tor/ticket/24487
*/
static smartlist_t *
build_middle_exclude_list(uint8_t purpose,
@ -2463,32 +2507,21 @@ build_middle_exclude_list(uint8_t purpose,
crypt_path_t *cpath;
int i;
/** Vanguard circuits have their own path selection rules */
if (circuit_should_use_vanguards(purpose)) {
return build_vanguard_middle_exclude_list(purpose, state, head, cur_len);
}
excluded = smartlist_new();
/* Add the exit to the exclude list (note that the exit/last hop is always
* chosen first in circuit_establish_circuit()). */
/* For non-vanguard circuits, add the exit and its family to the exclude list
* (note that the exit/last hop is always chosen first in
* circuit_establish_circuit()). */
if ((r = build_state_get_exit_node(state))) {
nodelist_add_node_and_family(excluded, r);
}
/* XXX: We don't apply any other previously selected node restrictions for
* vanguards, and allow nodes to be reused for those hop positions in the
* same circuit. This is because after many rotations, you get to learn
* inner guard nodes through the nodes that are not selected for outer
* hops.
*
* The alternative is building the circuit in reverse. Reverse calls to
* onion_extend_cpath() (ie: select outer hops first) would then have the
* property that you don't gain information about inner hops by observing
* outer ones. See https://trac.torproject.org/projects/tor/ticket/24487
* for this.
*
* (Note further that we can and do still exclude the exit in the block
* above, because it is chosen first in circuit_establish_circuit()..) */
if (circuit_should_use_vanguards(purpose)) {
return excluded;
}
/* also exclude all other already chosen nodes and their family */
for (i = 0, cpath = head; cpath && i < cur_len; ++i, cpath=cpath->next) {
if ((r = node_get_by_id(cpath->extend_info->identity_digest))) {
nodelist_add_node_and_family(excluded, r);