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      1 /*
      2  * Copyright (C) 2011 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 #include <cutils/uevent.h>
     18 
     19 #include <errno.h>
     20 #include <stdbool.h>
     21 #include <string.h>
     22 #include <strings.h>
     23 #include <sys/socket.h>
     24 #include <sys/un.h>
     25 #include <unistd.h>
     26 
     27 #include <linux/netlink.h>
     28 
     29 /**
     30  * Like recv(), but checks that messages actually originate from the kernel.
     31  */
     32 ssize_t uevent_kernel_multicast_recv(int socket, void *buffer, size_t length)
     33 {
     34     uid_t uid = -1;
     35     return uevent_kernel_multicast_uid_recv(socket, buffer, length, &uid);
     36 }
     37 
     38 /**
     39  * Like the above, but passes a uid_t in by pointer. In the event that this
     40  * fails due to a bad uid check, the uid_t will be set to the uid of the
     41  * socket's peer.
     42  *
     43  * If this method rejects a netlink message from outside the kernel, it
     44  * returns -1, sets errno to EIO, and sets "user" to the UID associated with the
     45  * message. If the peer UID cannot be determined, "user" is set to -1."
     46  */
     47 ssize_t uevent_kernel_multicast_uid_recv(int socket, void *buffer, size_t length, uid_t *uid)
     48 {
     49     return uevent_kernel_recv(socket, buffer, length, true, uid);
     50 }
     51 
     52 ssize_t uevent_kernel_recv(int socket, void *buffer, size_t length, bool require_group, uid_t *uid)
     53 {
     54     struct iovec iov = { buffer, length };
     55     struct sockaddr_nl addr;
     56     char control[CMSG_SPACE(sizeof(struct ucred))];
     57     struct msghdr hdr = {
     58         &addr,
     59         sizeof(addr),
     60         &iov,
     61         1,
     62         control,
     63         sizeof(control),
     64         0,
     65     };
     66 
     67     *uid = -1;
     68     ssize_t n = recvmsg(socket, &hdr, 0);
     69     if (n <= 0) {
     70         return n;
     71     }
     72 
     73     struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr);
     74     if (cmsg == NULL || cmsg->cmsg_type != SCM_CREDENTIALS) {
     75         /* ignoring netlink message with no sender credentials */
     76         goto out;
     77     }
     78 
     79     struct ucred *cred = (struct ucred *)CMSG_DATA(cmsg);
     80     *uid = cred->uid;
     81     if (cred->uid != 0) {
     82         /* ignoring netlink message from non-root user */
     83         goto out;
     84     }
     85 
     86     if (addr.nl_pid != 0) {
     87         /* ignore non-kernel */
     88         goto out;
     89     }
     90     if (require_group && addr.nl_groups == 0) {
     91         /* ignore unicast messages when requested */
     92         goto out;
     93     }
     94 
     95     return n;
     96 
     97 out:
     98     /* clear residual potentially malicious data */
     99     bzero(buffer, length);
    100     errno = EIO;
    101     return -1;
    102 }
    103 
    104 int uevent_open_socket(int buf_sz, bool passcred)
    105 {
    106     struct sockaddr_nl addr;
    107     int on = passcred;
    108     int s;
    109 
    110     memset(&addr, 0, sizeof(addr));
    111     addr.nl_family = AF_NETLINK;
    112     addr.nl_pid = getpid();
    113     addr.nl_groups = 0xffffffff;
    114 
    115     s = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC, NETLINK_KOBJECT_UEVENT);
    116     if(s < 0)
    117         return -1;
    118 
    119     setsockopt(s, SOL_SOCKET, SO_RCVBUFFORCE, &buf_sz, sizeof(buf_sz));
    120     setsockopt(s, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on));
    121 
    122     if(bind(s, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
    123         close(s);
    124         return -1;
    125     }
    126 
    127     return s;
    128 }
    129