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这篇文章是上一篇博客的后续分析，主要介绍向init进程发送热拔插信息后init进程的处理流程
首先我们来了解一个数据结构，uevent，如下
struct uevent {
const char *action;
const char *path;
const char *subsystem;
const char *firmware;
int major;
int minor;
};
内核收到的信息如下，ACTION=addDEVPATH=/../data/local/tmpSUBSYSTEM=firmwareFIRMWARE=../../../data/local/tmp/hotplug
通过如下函数parse_event进行解析
static void parse_event(const char *msg, struct uevent *uevent)
{
while(*msg) {
if(!strncmp(msg, "ACTION=", 7)) {
msg += 7;
uevent->action = msg;
} else if(!strncmp(msg, "DEVPATH=", 8)) {
msg += 8;
uevent->path = msg;
} else if(!strncmp(msg, "SUBSYSTEM=", 10)) {
msg += 10;
uevent->subsystem = msg;
} else if(!strncmp(msg, "FIRMWARE=", 9)) {
msg += 9;
uevent->firmware = msg;
} else if(!strncmp(msg, "MAJOR=", 6)) {
msg += 6;
uevent->major = atoi(msg);
} else if(!strncmp(msg, "MINOR=", 6)) {
msg += 6;
uevent->minor = atoi(msg);
}
while(*msg++)；
}
}
经过解析之后，uevent的结构为：
action="add"
path="/../data/local/tmp"
subsystem="firmware"
firmware="../../../data/local/tmp/hotplug"
之后来到处理firmware的核心函数
static void process_firmware_event(struct uevent *uevent)
{
l = asprintf(&root, SYSFS_PREFIX"%s/", uevent->path);
//root为/sys/../data/local/tmp/=/data/local/tmp/
l = asprintf(&loading, "%sloading", root);
//loading为/data/local/tmp/loading
l = asprintf(&data, "%sdata", root);
//data为/data/local/tmp/data 其内容为指向/proc/sys/kernel/hotplug的符号链接
l = asprintf(&file, FIRMWARE_DIR"/%s", uevent->firmware);
//file为/etc/firmware/../../../data/local/tmp/hotplug=/data/local/tmp/hotplug
loading_fd = open(loading, O_WRONLY);
data_fd = open(data, O_WRONLY);
fw_fd = open(file, O_RDONLY);
load_firmware(fw_fd, loading_fd, data_fd)；
}
最后来到load_firmware函数,把hotplug中的数据写到/proc/sys/kernel/hotplug中
其内容变为/data/local/tmp/exploid
static int load_firmware(int fw_fd, int loading_fd, int data_fd)
{
while (len_to_copy > 0) {
char buf[PAGE_SIZE];
nr = read(fw_fd, buf, sizeof(buf));
len_to_copy -= nr;
while (nr > 0) {
nw = write(data_fd, buf + nw, nr);
nr -= nw;
}
}
}终于/proc/sys/kernel/hotplug中写入了我们的恶意程序了，只要再次受到如wifi打开、usb插入等热拔插信息，内核就会以root权限加载我们的程序再一次执行，从而达到提权的目的
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