text: /*
   * Copyright (C) 2011 The Android Open Source Project
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  #include <cutils/uevent.h>
  
  #include <errno.h>
  #include <strings.h>
  
  #include <linux/netlink.h>
  
  /**
   * Like recv(), but checks that messages actually originate from the kernel.
   */
  ssize_t uevent_checked_recv(int socket, void *buffer, size_t length) {
      struct iovec iov = { buffer, length };
      struct sockaddr_nl addr;
      char control[CMSG_SPACE(sizeof(struct ucred))];
      struct msghdr hdr = {
          &addr,
          sizeof(addr),
          &iov,
          1,
          control,
          sizeof(control),
          0,
      };
  
      ssize_t n = recvmsg(socket, &hdr, 0);
      if (n <= 0) {
          return n;
      }
  
      if (addr.nl_groups == 0 || addr.nl_pid != 0) {
          /* ignoring non-kernel or unicast netlink message */
          goto out;
      }
  
      struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr);
      if (cmsg == NULL || cmsg->cmsg_type != SCM_CREDENTIALS) {
          /* ignoring netlink message with no sender credentials */
          goto out;
      }
  
      struct ucred *cred = (struct ucred *)CMSG_DATA(cmsg);
      if (cred->uid != 0) {
          /* ignoring netlink message from non-root user */
          goto out;
      }
  
      return n;
  
  out:
      /* clear residual potentially malicious data */
      bzero(buffer, length);
      errno = EIO;
      return -1;
  }
  