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 */
32ssize_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 */
47ssize_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
52ssize_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
97out:
98    /* clear residual potentially malicious data */
99    bzero(buffer, length);
100    errno = EIO;
101    return -1;
102}
103
104int 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