protover: TROVE-2018-005 Fix potential DoS in protover protocol parsing.

In protover.c, the `expand_protocol_list()` function expands a `smartlist_t` of
`proto_entry_t`s to their protocol name concatenated with each version number.
For example, given a `proto_entry_t` like so:

    proto_entry_t *proto = tor_malloc(sizeof(proto_entry_t));
    proto_range_t *range = tor_malloc_zero(sizeof(proto_range_t));

    proto->name = tor_strdup("DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa");
    proto->ranges = smartlist_new();

    range->low = 1;
    range->high = 65536;

    smartlist_add(proto->ranges, range);

(Where `[19KB]` is roughly 19KB of `"a"` bytes.)  This would expand in
`expand_protocol_list()` to a `smartlist_t` containing 65536 copies of the
string, e.g.:

    "DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa=1"
    "DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa=2"
    […]
    "DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa=65535"

Thus constituting a potential resource exhaustion attack.

The Rust implementation is not subject to this attack, because it instead
expands the above string into a `HashMap<String, HashSet<u32>` prior to #24031,
and a `HashMap<UnvalidatedProtocol, ProtoSet>` after).  Neither Rust version is
subject to this attack, because it only stores the `String` once per protocol.
(Although a related, but apparently of too minor impact to be usable, DoS bug
has been fixed in #24031. [0])

[0]: https://bugs.torproject.org/24031

 * ADDS hard limit on protocol name lengths in protover.c and checks in
   parse_single_entry() and expand_protocol_list().
 * ADDS tests to ensure the bug is caught.
 * FIXES #25517: https://bugs.torproject.org/25517

1 files changed, 22 insertions, 0 deletions

diff --git a/src/or/protover.c b/src/or/protover.c
index 45f0377d61..24b3813614 100644
--- a/src/or/protover.c
+++ b/src/or/protover.c
@@ -50,6 +50,11 @@ static const struct {
 
 #define N_PROTOCOL_NAMES ARRAY_LENGTH(PROTOCOL_NAMES)
 
+/* Maximum allowed length of any single subprotocol name. */
+// C_RUST_COUPLED: src/rust/protover/protover.rs
+//                 `MAX_PROTOCOL_NAME_LENGTH`
+static const uint MAX_PROTOCOL_NAME_LENGTH = 100;
+
 /**
  * Given a protocol_type_t, return the corresponding string used in
  * descriptors.
@@ -195,6 +200,15 @@ parse_single_entry(const char *s, const char *end_of_entry)
   if (equals == s)
     goto error;
 
+  /* The name must not be longer than MAX_PROTOCOL_NAME_LENGTH. */
+  if (equals - s > MAX_PROTOCOL_NAME_LENGTH) {
+    log_warn(LD_NET, "When parsing a protocol entry, I got a very large "
+             "protocol name. This is possibly an attack or a bug, unless "
+             "the Tor network truly supports protocol names larger than "
+             "%ud characters. The offending string was: %s",
+             MAX_PROTOCOL_NAME_LENGTH, escaped(out->name));
+    goto error;
+  }
   out->name = tor_strndup(s, equals-s);
 
   tor_assert(equals < end_of_entry);
@@ -397,6 +411,14 @@ expand_protocol_list(const smartlist_t *protos)
 
   SMARTLIST_FOREACH_BEGIN(protos, const proto_entry_t *, ent) {
     const char *name = ent->name;
+    if (strlen(name) > MAX_PROTOCOL_NAME_LENGTH) {
+      log_warn(LD_NET, "When expanding a protocol entry, I got a very large "
+               "protocol name. This is possibly an attack or a bug, unless "
+               "the Tor network truly supports protocol names larger than "
+               "%ud characters. The offending string was: %s",
+               MAX_PROTOCOL_NAME_LENGTH, escaped(name));
+      continue;
+    }
     SMARTLIST_FOREACH_BEGIN(ent->ranges, const proto_range_t *, range) {
       uint32_t u;
       for (u = range->low; u <= range->high; ++u) {