the kernel libproc API proc_list_uptrs has the following comment in it's userspace header:

/*
 * Enumerate potential userspace pointers embedded in kernel data structures.
 * Currently inspects kqueues only.
 *
 * NOTE: returned "pointers" are opaque user-supplied values and thus not
 * guaranteed to address valid objects or be pointers at all.
 *
 * Returns the number of pointers found (which may exceed buffersize), or -1 on
 * failure and errno set appropriately.
 */

This is a recent addition to the kernel, presumably as a debugging tool to help enumerate
places where the kernel is accidentally disclosing kernel pointers to userspace.

The implementation currently enumerates kqueues and dumps a bunch of values from them.

Here's the relevant code:

// buffer and buffersize are attacker controlled

int
proc_pidlistuptrs(proc_t p, user_addr_t buffer, uint32_t buffersize, int32_t *retval)
{
  uint32_t count = 0;
  int error = 0;
  void *kbuf = NULL;
  int32_t nuptrs = 0;

  if (buffer != USER_ADDR_NULL) {
    count = buffersize / sizeof(uint64_t);     <---(a)
    if (count > MAX_UPTRS) {
      count = MAX_UPTRS;
      buffersize = count * sizeof(uint64_t);
    }
    if (count > 0) {
      kbuf = kalloc(buffersize);               <--- (b)
      assert(kbuf != NULL);
    }
  } else {
    buffersize = 0;
  }

  nuptrs = kevent_proc_copy_uptrs(p, kbuf, buffersize);

  if (kbuf) {
    size_t copysize;
    if (os_mul_overflow(nuptrs, sizeof(uint64_t), &copysize)) {  <--- (c)
      error = ERANGE;
      goto out;
    }
    if (copysize > buffersize) {    <-- (d)
      copysize = buffersize;
    }
    error = copyout(kbuf, buffer, copysize);  <--- (e)
  }


At (a) the attacker-supplied buffersize is divided by 8 to compute the maximum number of uint64_t's
which can fit in there.

If that value isn't huge then the attacker-supplied buffersize is used to kalloc the kbuf buffer at (b).

kbuf and buffersize are then passed to kevent_proc_copy_uptrs. Looking at the implementation of
kevent_proc_copy_uptrs the return value is the total number of values it found, even if that value is larger
than the supplied buffer. If it finds more than will fit it keeps counting but no longer writes them to the kbuf.

This means that at (c) the computed copysize value doesn't reflect how many values were actually written to kbuf
but how many *could* have been written had the buffer been big enough.

If there were possible values which could have been written than there was space in the buffer then at (d) copysize
will be limited down to buffersize.

Copysize is then used at (e) to copy the contents of kbuf to userspace.

The bug is that there's no enforcement that (buffersize % 8) == 0. If we were to pass a buffersize of 15, at (a) count would be 1
as 15 bytes is only enough to store 1 complete uint64_t. At (b) this would kalloc a buffer of 15 bytes.

If the target pid actually had 10 possible values which kevent_proc_copy_uptrs finds then nuptrs will return 10 but it will
only write to the first value to kbuf, leaving the last 7 bytes untouched.

At (c) copysize will be computed at 10*8 = 80 bytes, at (d) since 80 > 15 copysize will be truncated back down to buffersize (15)
and at (e) 15 bytes will be copied back to userspace even though only 8 were written to.

Kalloc doesn't zero-initialise returned memory so this can be used to easily and safely disclose lots of kernel memory, albeit
limited to the 7-least significant bytes of each 8-byte aligned qword. That's more than enough to easily defeat kaslr.

This PoC demonstrates the disclosure of kernel pointers in the stale kalloc memory.

Tested on MacOS 10.13 High Sierra (17A365)
 

uptrs.c 4.5 KB View Download