Quelle kcmp.c Sprache: C

// SPDX-License-Identifier: GPL-2.0
#include <linux/kernel.h>
#include <linux/syscalls.h>
#include <linux/fdtable.h>
#include <linux/string.h>
#include <linux/random.h>
#include <linux/module.h>
#include <linux/ptrace.h>
#include <linux/init.h>
#include <linux/errno.h>
#include <linux/cache.h>
#include <linux/bug.h>
#include <linux/err.h>
#include <linux/kcmp.h>
#include <linux/capability.h>
#include <linux/list.h>
#include <linux/eventpoll.h>
#include <linux/file.h>

#include <asm/unistd.h>

/*
* We don't expose the real in-memory order of objects for security reasons.
* But still the comparison results should be suitable for sorting. So we
* obfuscate kernel pointers values and compare the production instead.
*
* The obfuscation is done in two steps. First we xor the kernel pointer with
* a random value, which puts pointer into a new position in a reordered space.
* Secondly we multiply the xor production with a large odd random number to
* permute its bits even more (the odd multiplier guarantees that the product
* is unique ever after the high bits are truncated, since any odd number is
* relative prime to 2^n).
*
* Note also that the obfuscation itself is invisible to userspace and if needed
* it can be changed to an alternate scheme.
*/
static unsigned long cookies[KCMP_TYPES][2] __read_mostly;

static long kptr_obfuscate(long v, int type)
{
return (v ^ cookies[type][0]) * cookies[type][1];
}

/*
* 0 - equal, i.e. v1 = v2
* 1 - less than, i.e. v1 < v2
* 2 - greater than, i.e. v1 > v2
* 3 - not equal but ordering unavailable (reserved for future)
*/
static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
{
long t1, t2;

t1 = kptr_obfuscate((long)v1, type);
t2 = kptr_obfuscate((long)v2, type);

return (t1 < t2) | ((t1 > t2) << 1);
}

/* The caller must have pinned the task */
static struct file *
get_file_raw_ptr(struct task_struct *task, unsigned int idx)
{
struct file *file;

file = fget_task(task, idx);
if (file)
  fput(file);

return file;
}

static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
{
if (likely(l2 != l1))
  up_read(l2);
up_read(l1);
}

static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
{
int err;

if (l2 > l1)
  swap(l1, l2);

err = down_read_killable(l1);
if (!err && likely(l1 != l2)) {
  err = down_read_killable_nested(l2, SINGLE_DEPTH_NESTING);
  if (err)
   up_read(l1);
}

return err;
}

#ifdef CONFIG_EPOLL
static int kcmp_epoll_target(struct task_struct *task1,
        struct task_struct *task2,
        unsigned long idx1,
        struct kcmp_epoll_slot __user *uslot)
{
struct file *filp, *filp_epoll, *filp_tgt;
struct kcmp_epoll_slot slot;

if (copy_from_user(&slot, uslot, sizeof(slot)))
  return -EFAULT;

filp = get_file_raw_ptr(task1, idx1);
if (!filp)
  return -EBADF;

filp_epoll = fget_task(task2, slot.efd);
if (!filp_epoll)
  return -EBADF;

filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
fput(filp_epoll);

if (IS_ERR(filp_tgt))
  return PTR_ERR(filp_tgt);

return kcmp_ptr(filp, filp_tgt, KCMP_FILE);
}
#else
static int kcmp_epoll_target(struct task_struct *task1,
        struct task_struct *task2,
        unsigned long idx1,
        struct kcmp_epoll_slot __user *uslot)
{
return -EOPNOTSUPP;
}
#endif

SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
  unsigned long, idx1, unsigned long, idx2)
{
struct task_struct *task1, *task2;
int ret;

rcu_read_lock();

/*
* Tasks are looked up in caller's PID namespace only.
*/
task1 = find_task_by_vpid(pid1);
task2 = find_task_by_vpid(pid2);
if (unlikely(!task1 || !task2))
  goto err_no_task;

get_task_struct(task1);
get_task_struct(task2);

rcu_read_unlock();

/*
* One should have enough rights to inspect task details.
*/
ret = kcmp_lock(&task1->signal->exec_update_lock,
   &task2->signal->exec_update_lock);
if (ret)
  goto err;
if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
     !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
  ret = -EPERM;
  goto err_unlock;
}

switch (type) {
case KCMP_FILE: {
  struct file *filp1, *filp2;

  filp1 = get_file_raw_ptr(task1, idx1);
  filp2 = get_file_raw_ptr(task2, idx2);

  if (filp1 && filp2)
   ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
  else
   ret = -EBADF;
  break;
}
case KCMP_VM:
  ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
  break;
case KCMP_FILES:
  ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
  break;
case KCMP_FS:
  ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
  break;
case KCMP_SIGHAND:
  ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
  break;
case KCMP_IO:
  ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
  break;
case KCMP_SYSVSEM:
#ifdef CONFIG_SYSVIPC
  ret = kcmp_ptr(task1->sysvsem.undo_list,
          task2->sysvsem.undo_list,
          KCMP_SYSVSEM);
#else
  ret = -EOPNOTSUPP;
#endif
  break;
case KCMP_EPOLL_TFD:
  ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
  break;
default:
  ret = -EINVAL;
  break;
}

err_unlock:
kcmp_unlock(&task1->signal->exec_update_lock,
      &task2->signal->exec_update_lock);
err:
put_task_struct(task1);
put_task_struct(task2);

return ret;

err_no_task:
rcu_read_unlock();
return -ESRCH;
}

static __init int kcmp_cookies_init(void)
{
int i;

get_random_bytes(cookies, sizeof(cookies));

for (i = 0; i < KCMP_TYPES; i++)
  cookies[i][1] |= (~(~0UL >>  1) | 1);

return 0;
}
arch_initcall(kcmp_cookies_init);

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