uevent.c revision 57de8b8f27f887ca8084671df777a4ac199ba647 
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 <strings.h>
21
22#include <linux/netlink.h>
23
24/**
25 * Like recv(), but checks that messages actually originate from the kernel.
26 */
27ssize_t uevent_kernel_multicast_recv(int socket, void *buffer, size_t length) {
28    struct iovec iov = { buffer, length };
29    struct sockaddr_nl addr;
30    char control[CMSG_SPACE(sizeof(struct ucred))];
31    struct msghdr hdr = {
32        &addr,
33        sizeof(addr),
34        &iov,
35        1,
36        control,
37        sizeof(control),
38        0,
39    };
40
41    ssize_t n = recvmsg(socket, &hdr, 0);
42    if (n <= 0) {
43        return n;
44    }
45
46    if (addr.nl_groups == 0 || addr.nl_pid != 0) {
47        /* ignoring non-kernel or unicast netlink message */
48        goto out;
49    }
50
51    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr);
52    if (cmsg == NULL || cmsg->cmsg_type != SCM_CREDENTIALS) {
53        /* ignoring netlink message with no sender credentials */
54        goto out;
55    }
56
57    struct ucred *cred = (struct ucred *)CMSG_DATA(cmsg);
58    if (cred->uid != 0) {
59        /* ignoring netlink message from non-root user */
60        goto out;
61    }
62
63    return n;
64
65out:
66    /* clear residual potentially malicious data */
67    bzero(buffer, length);
68    errno = EIO;
69    return -1;
70}
71