prop224: Set stricter expiration time of cached client descriptors.

In #23466 we discovered that cached descriptors can stay around on the
client-side for up to 72 hours. In reality we only want those descs to
get cached for the duration of the current time period, since after that
TP is gone the client needs to compute a new blinded key to use for the HS.

In this commit we start using the consensus time (if available) when
cleaning up cached client descriptor entries. That makes sense because
the client uses consensus time anyway for connecting to hidden
services (e.g. computing blinded keys and time periods).

If no recent consensus is available, we consider descriptors to be
expired since we will want to fetch new ones when we get a live
consensus to avoid the Roger bug. If we didn't do that, when Roger
desuspends his laptop there would be a race between Tor fetching a new
consensus, and Tor connecting to the HS which would still cause
reachability issues.

We also turned a rev counter check into a BUG, since we should never
receive a descriptor with a strictly smaller rev counter than the one we
already have, except if there is a bug or if the HSDir wants to mess
with us. In any case, let's turn this into a BUG so that we can detect
and debug such cases easily.

src/or/hs_cache.c

@@ -20,6 +20,9 @@
 
 #include "hs_cache.h"
 
+static int cached_client_descriptor_has_expired(time_t now,
+           const hs_cache_client_descriptor_t *cached_desc);
+
 /********************** Directory HS cache ******************/
 
 /* Directory descriptor cache. Map indexed by blinded key. */
@@ -356,12 +359,27 @@ store_v3_desc_as_client(hs_cache_client_descriptor_t *desc)
   rend_cache_increment_allocation(cache_get_client_entry_size(desc));
 }
 
-/* Query our cache and return the entry or NULL if not found. */
+/* Query our cache and return the entry or NULL if not found or if expired. */
 STATIC hs_cache_client_descriptor_t *
 lookup_v3_desc_as_client(const uint8_t *key)
 {
+  time_t now = approx_time();
+  hs_cache_client_descriptor_t *cached_desc;
+
   tor_assert(key);
-  return digest256map_get(hs_cache_v3_client, key);
+
+  /* Do the lookup */
+  cached_desc = digest256map_get(hs_cache_v3_client, key);
+  if (!cached_desc) {
+    return NULL;
+  }
+
+  /* Don't return expired entries */
+  if (cached_client_descriptor_has_expired(now, cached_desc)) {
+    return NULL;
+  }
+
+  return cached_desc;
 }
 
 /* Parse the encoded descriptor in <b>desc_str</b> using
@@ -388,7 +406,10 @@ cache_client_desc_new(const char *desc_str,
   /* All is good: make a cache object for this descriptor */
   client_desc = tor_malloc_zero(sizeof(hs_cache_client_descriptor_t));
   ed25519_pubkey_copy(&client_desc->key, service_identity_pk);
-  client_desc->created_ts = approx_time();
+  /* Set expiration time for this cached descriptor to be the start of the next
+   * time period since that's when clients need to start using the next blinded
+   * pk of the service (and hence will need its next descriptor). */
+  client_desc->expiration_ts = hs_get_start_time_of_next_time_period(0);
   client_desc->desc = desc;
   client_desc->encoded_desc = tor_strdup(desc_str);
 
@@ -603,9 +624,8 @@ cache_store_as_client(hs_cache_client_descriptor_t *client_desc)
   if (cache_entry != NULL) {
     /* If we have an entry in our cache that has a revision counter greater
      * than the one we just fetched, discard the one we fetched. */
-    if (cache_entry->desc->plaintext_data.revision_counter >
-        client_desc->desc->plaintext_data.revision_counter) {
-      log_info(LD_REND, "We already have fresher descriptor. Ignoring.");
+    if (BUG(cache_entry->desc->plaintext_data.revision_counter >
+            client_desc->desc->plaintext_data.revision_counter)) {
       cache_client_desc_free(client_desc);
       goto done;
     }
@@ -621,7 +641,30 @@ cache_store_as_client(hs_cache_client_descriptor_t *client_desc)
   return 0;
 }
 
-/* Clean the client cache using now as the current time. Return the total size
+/* Return true iff the cached client descriptor at <b>cached_desc</b has
+ * expired. */
+static int
+cached_client_descriptor_has_expired(time_t now,
+                               const hs_cache_client_descriptor_t *cached_desc)
+{
+  /* We use the current consensus time to see if we should expire this
+   * descriptor since we use consensus time for all other parts of the protocol
+   * as well (e.g. to build the blinded key and compute time periods). */
+  const networkstatus_t *ns = networkstatus_get_live_consensus(now);
+  /* If we don't have a recent consensus, consider this entry expired since we
+   * will want to fetch a new HS desc when we get a live consensus. */
+  if (!ns) {
+    return 1;
+  }
+
+  if (cached_desc->expiration_ts <= ns->valid_after) {
+    return 1;
+  }
+
+  return 0;
+}
+
+/* clean the client cache using now as the current time. Return the total size
  * of removed bytes from the cache. */
 static size_t
 cache_clean_v3_as_client(time_t now)
@@ -635,11 +678,9 @@ cache_clean_v3_as_client(time_t now)
   DIGEST256MAP_FOREACH_MODIFY(hs_cache_v3_client, key,
                               hs_cache_client_descriptor_t *, entry) {
     size_t entry_size;
-    time_t cutoff = now - rend_cache_max_entry_lifetime();
 
-    /* If the entry has been created _after_ the cutoff, not expired so
-     * continue to the next entry in our v3 cache. */
-    if (entry->created_ts > cutoff) {
+    /* If the entry has not expired, continue to the next cached entry */
+    if (!cached_client_descriptor_has_expired(now, entry)) {
       continue;
     }
     /* Here, our entry has expired, remove and free. */

src/or/hs_cache.h

@@ -103,8 +103,10 @@ typedef struct hs_cache_client_descriptor_t {
   /* This object is indexed using the service identity public key */
   ed25519_public_key_t key;
 
-  /* When was this entry created. Used to expire entries. */
-  time_t created_ts;
+  /* When will this entry expire? We expire cached client descriptors in the
+   * start of the next time period, since that's when clients need to start
+   * using the next blinded key of the service. */
+  time_t expiration_ts;
 
   /* The cached descriptor, this object is the owner. It can't be NULL. A
    * cache object without a valid descriptor is not possible. */