linux/kernel/kcmp.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)
{ … }

/*
 * 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)
{ … }

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

static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
{ … }

static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
{ … }

#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)
{ … }
#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)
{ … }

static __init int kcmp_cookies_init(void)
{ … }
arch_initcall(kcmp_cookies_init);