libcutils/uevent.cpp

/*
 * 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 <stdint.h>
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>

#include <linux/netlink.h>

#include <fstream>

#include <private/android_filesystem_config.h>

namespace {

// Returns the uid of root in the current user namespace.
// Returns AID_OVERFLOWUID if the root user is not mapped in the current
// namespace.
// Returns 0 if the kernel is not user namespace-aware (for backwards
// compatibility) or if AID_OVERFLOWUID could not be validated to match what the
// kernel would return.
uid_t GetRootUid() {
    constexpr uid_t kParentRootUid = 0;

    std::ifstream uid_map_file("/proc/self/uid_map");
    if (!uid_map_file) {
        // The kernel does not support user namespaces.
        return kParentRootUid;
    }

    uid_t current_namespace_uid, parent_namespace_uid;
    uint32_t length;
    while (uid_map_file >> current_namespace_uid >> parent_namespace_uid >> length) {
        // Since kParentRootUid is 0, it should be the first entry in the mapped
        // range.
        if (parent_namespace_uid != kParentRootUid || length < 1) continue;
        return current_namespace_uid;
    }

    // Sanity check: verify that the overflow UID is the one to be returned by
    // the kernel.
    std::ifstream overflowuid_file("/proc/sys/kernel/overflowuid");
    if (!overflowuid_file) {
        // It's better to return 0 in case we cannot make sure that the overflow
        // UID matches.
        return kParentRootUid;
    }
    uid_t kernel_overflow_uid;
    if (!(overflowuid_file >> kernel_overflow_uid) || kernel_overflow_uid != AID_OVERFLOWUID)
        return kParentRootUid;

    // root is unmapped, use the kernel "overflow" uid.
    return AID_OVERFLOWUID;
}

}  // namespace

extern "C" {

/**
 * Like recv(), but checks that messages actually originate from the kernel.
 */
ssize_t uevent_kernel_multicast_recv(int socket, void* buffer, size_t length) {
    uid_t uid = -1;
    return uevent_kernel_multicast_uid_recv(socket, buffer, length, &uid);
}

/**
 * Like the above, but passes a uid_t in by pointer. In the event that this
 * fails due to a bad uid check, the uid_t will be set to the uid of the
 * socket's peer.
 *
 * If this method rejects a netlink message from outside the kernel, it
 * returns -1, sets errno to EIO, and sets "user" to the UID associated with the
 * message. If the peer UID cannot be determined, "user" is set to -1."
 */
ssize_t uevent_kernel_multicast_uid_recv(int socket, void* buffer, size_t length, uid_t* uid) {
    return uevent_kernel_recv(socket, buffer, length, true, uid);
}

ssize_t uevent_kernel_recv(int socket, void* buffer, size_t length, bool require_group, uid_t* uid) {
    static const uid_t root_uid = GetRootUid();
    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,
    };
    struct ucred* cred;

    *uid = -1;
    ssize_t n = recvmsg(socket, &hdr, 0);
    if (n <= 0) {
        return n;
    }

    struct cmsghdr* cmsg = CMSG_FIRSTHDR(&hdr);
    if (cmsg == NULL || cmsg->cmsg_type != SCM_CREDENTIALS) {
        /* ignoring netlink message with no sender credentials */
        goto out;
    }

    cred = (struct ucred*)CMSG_DATA(cmsg);
    *uid = cred->uid;
    if (cred->uid != root_uid) {
        /* ignoring netlink message from non-root user */
        goto out;
    }

    if (addr.nl_pid != 0) {
        /* ignore non-kernel */
        goto out;
    }
    if (require_group && addr.nl_groups == 0) {
        /* ignore unicast messages when requested */
        goto out;
    }

    return n;

out:
    /* clear residual potentially malicious data */
    bzero(buffer, length);
    errno = EIO;
    return -1;
}

int uevent_open_socket(int buf_sz, bool passcred) {
    struct sockaddr_nl addr;
    int on = passcred;
    int s;

    memset(&addr, 0, sizeof(addr));
    addr.nl_family = AF_NETLINK;
    addr.nl_pid = getpid();
    addr.nl_groups = 0xffffffff;

    s = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC, NETLINK_KOBJECT_UEVENT);
    if (s < 0) return -1;

    /* buf_sz should be less than net.core.rmem_max for this to succeed */
    if (setsockopt(s, SOL_SOCKET, SO_RCVBUF, &buf_sz, sizeof(buf_sz)) < 0) {
        close(s);
        return -1;
    }

    setsockopt(s, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on));

    if (bind(s, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        close(s);
        return -1;
    }

    return s;
}

}  // extern "C"