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Remove DoS vector in protover.c voting code

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This commit is contained in:
Nick Mathewson 2016-08-26 12:54:41 -04:00
parent e525f5697f
commit 1e29c68ba9
1 changed files with 25 additions and 8 deletions
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29
src/or/protover.c
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@ -337,18 +337,22 @@ encode_protocol_list(const smartlist_t *sl)
return result;
}
/* We treat any protocol list with more than this many subprotocols in it
* as a DoS attempt. */
const int MAX_PROTOCOLS_TO_EXPAND = (1<<16);
/** Voting helper: Given a list of proto_entry_t, return a newly allocated
* smartlist of newly allocated strings, one for each included protocol
* version. (So 'Foo=3,5-7' expands to a list of 'Foo=3', 'Foo=5', 'Foo=6',
* 'Foo=7'.)
*
* Do not list any protocol version more than once. */
* Do not list any protocol version more than once.
*
* Return NULL if the list would be too big.
*/
static smartlist_t *
expand_protocol_list(const smartlist_t *protos)
{
// XXXX This can make really huge lists from small inputs; that's a DoS
// problem.
smartlist_t *expanded = smartlist_new();
if (!protos)
return expanded;
@ -359,6 +363,8 @@ expand_protocol_list(const smartlist_t *protos)
uint32_t u;
for (u = range->low; u <= range->high; ++u) {
smartlist_add_asprintf(expanded, "%s=%lu", name, (unsigned long)u);
if (smartlist_len(expanded) > MAX_PROTOCOLS_TO_EXPAND)
goto too_many;
}
} SMARTLIST_FOREACH_END(range);
} SMARTLIST_FOREACH_END(ent);
@ -366,6 +372,11 @@ expand_protocol_list(const smartlist_t *protos)
smartlist_sort_strings(expanded);
smartlist_uniq_strings(expanded); // This makes voting work. do not remove
return expanded;
too_many:
SMARTLIST_FOREACH(expanded, char *, cp, tor_free(cp));
smartlist_free(expanded);
return NULL;
}
/** Voting helper: compare two singleton proto_entry_t items by version
@ -512,16 +523,22 @@ char *
compute_protover_vote(const smartlist_t *list_of_proto_strings,
int threshold)
{
// XXXX This algorithm can be made to use too much RAM. Fix that.
smartlist_t *all_entries = smartlist_new();
// First, parse the inputs and break them into singleton entries.
SMARTLIST_FOREACH_BEGIN(list_of_proto_strings, const char *, vote) {
smartlist_t *unexpanded = parse_protocol_list(vote);
smartlist_t *this_vote = expand_protocol_list(unexpanded);
if (this_vote == NULL) {
log_warn(LD_NET, "When expanding a protocol list from an authority, I "
"got too many protocols. This is possibly an attack or a bug, "
"unless the Tor network truly has expanded to support over %d "
"different subprotocol versions. The offending string was: %s",
MAX_PROTOCOLS_TO_EXPAND, escaped(vote));
} else {
smartlist_add_all(all_entries, this_vote);
smartlist_free(this_vote);
}
SMARTLIST_FOREACH(unexpanded, proto_entry_t *, e, proto_entry_free(e));
smartlist_free(unexpanded);
} SMARTLIST_FOREACH_END(vote);