MacOS kernel memory corruption due to off-by-one in audit_pipe_open

audit_pipe_open is the special file open handler for the auditpipe device (major number 10.)

Here's the code:

  static int
  audit_pipe_open(dev_t dev, __unused int flags,  __unused int devtype,
      __unused proc_t p)
  {
    struct audit_pipe *ap;
    int u;

    u = minor(dev);
    if (u < 0 || u > MAX_AUDIT_PIPES)
      return (ENXIO);

    AUDIT_PIPE_LIST_WLOCK();
    ap = audit_pipe_dtab[u];
    if (ap == NULL) {
      ap = audit_pipe_alloc();
      if (ap == NULL) {
        AUDIT_PIPE_LIST_WUNLOCK();
        return (ENOMEM);
      }
      audit_pipe_dtab[u] = ap;


We can control the minor number via mknod. Here's the definition of audit_pipe_dtab:

  static struct audit_pipe  *audit_pipe_dtab[MAX_AUDIT_PIPES];

There's an off-by-one in the minor number bounds check
 (u < 0 || u > MAX_AUDIT_PIPES)
should be
 (u < 0 || u >= MAX_AUDIT_PIPES)

The other special file operation handlers assume that the minor number of an opened device
is correct therefore it isn't validated for example in the ioctl handler:

  static int
  audit_pipe_ioctl(dev_t dev, u_long cmd, caddr_t data,
      __unused int flag, __unused proc_t p)
  {
    ...
    ap = audit_pipe_dtab[minor(dev)];
    KASSERT(ap != NULL, ("audit_pipe_ioctl: ap == NULL"));
    ...
    switch (cmd) {
    case FIONBIO:
      AUDIT_PIPE_LOCK(ap);
      if (*(int *)data)

Directly after the audit_pipe_dtab array in the bss is this global variable:

  static u_int64_t  audit_pipe_drops;

audit_pipe_drops will be incremented each time an audit message enqueue fails:

  if (ap->ap_qlen >= ap->ap_qlimit) {
    ap->ap_drops++;
    audit_pipe_drops++;
    return;
  }

So by setting a small ap_qlimit via the AUDITPIPE_SET_QLIMIT ioctl we can increment the
struct audit_pipe* which is read out-of-bounds.

For this PoC I mknod a /dev/auditpipe with the minor number 32, create a new log file
and enable auditing. I then set the QLIMIT to 1 and alternately enqueue a new audit record
and call and ioctl. Each time the enqueue fails it will increment the struct audit_pipe*
then the ioctl will try to use that pointer.

This is a root to kernel privesc.

tested on MacOS 10.12.3 (16D32) on MacbookAir5,2
 

auditpipe.c 3.2 KB View Download