Fix UAF in sdp_discovery.cc

It is possible with modifications to a client to open two connections
against the same SDP discovery database.  If this happens, it becomes
possible to reference a freed instance of the discovery database in the
second connection once the first one is closed.

To guard against this, check during discovery if a database has already
been allocated, and abort iff it has.

Also, add a null check to process_service_search_attr_rsp to guard
against unchecked calls to the SDP discovery database.

Bug: 291281168
Bug: 356201480
Flag: com.android.bluetooth.flags.btsec_check_valid_discovery_database
Test: atest bluetooth_test_gd_unit, net_test_stack_sdp
Tag: #security
Ignore-AOSP-First: Security
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c7bc0115a7cbad342628c8428af1b8181ee1ae94)
Merged-In: I754bf8292e1e0d8e90e78fa87889284e26aa5818
Change-Id: I754bf8292e1e0d8e90e78fa87889284e26aa5818

system/bta/hf_client/bta_hf_client_sdp.cc

@@ -319,6 +319,19 @@ void bta_hf_client_do_disc(tBTA_HF_CLIENT_CB* client_cb) {
     uuid_list[0] = Uuid::From16Bit(UUID_SERVCLASS_AG_HANDSFREE);
   }
 
+  /* If we already have a non-null discovery database at this point, we can get
+   * into a race condition leading to UAF once this connection is closed.
+   * This should only happen with malicious modifications to a client. */
+  if (client_cb->p_disc_db != NULL) {
+    APPL_TRACE_ERROR(
+        "Tried to set up a HF client with a preexisting discovery database.");
+    client_cb->p_disc_db = NULL;
+    // We manually set the state here because it's possible to call this from an
+    // OPEN state, in which case the discovery fail event will be ignored.
+    client_cb->state = 0;  // BTA_HF_CLIENT_INIT_ST
+    return;
+  }
+
   /* allocate buffer for sdp database */
   client_cb->p_disc_db = (tSDP_DISCOVERY_DB*)osi_malloc(BT_DEFAULT_BUFFER_SIZE);
 

system/stack/sdp/sdp_discovery.cc

@@ -596,6 +596,15 @@ static void process_service_search_attr_rsp(tCONN_CB* p_ccb, uint8_t* p_reply,
     uint8_t* p;
     uint16_t bytes_left = SDP_DATA_BUF_SIZE;
 
+    /* If we don't have a valid discovery database, we can't do anything. */
+    if (p_ccb->p_db == NULL) {
+      SDP_TRACE_WARNING(
+          "Attempted continuation or first time request with invalid discovery "
+          "database");
+      sdp_disconnect(p_ccb, tSDP_STATUS::SDP_INVALID_CONT_STATE);
+      return;
+    }
+
     p_msg->offset = L2CAP_MIN_OFFSET;
     p = p_start = (uint8_t*)(p_msg + 1) + L2CAP_MIN_OFFSET;