linux/kernel/kcmp.c
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   1#include <linux/kernel.h>
   2#include <linux/syscalls.h>
   3#include <linux/fdtable.h>
   4#include <linux/string.h>
   5#include <linux/random.h>
   6#include <linux/module.h>
   7#include <linux/init.h>
   8#include <linux/errno.h>
   9#include <linux/cache.h>
  10#include <linux/bug.h>
  11#include <linux/err.h>
  12#include <linux/kcmp.h>
  13
  14#include <asm/unistd.h>
  15
  16/*
  17 * We don't expose the real in-memory order of objects for security reasons.
  18 * But still the comparison results should be suitable for sorting. So we
  19 * obfuscate kernel pointers values and compare the production instead.
  20 *
  21 * The obfuscation is done in two steps. First we xor the kernel pointer with
  22 * a random value, which puts pointer into a new position in a reordered space.
  23 * Secondly we multiply the xor production with a large odd random number to
  24 * permute its bits even more (the odd multiplier guarantees that the product
  25 * is unique ever after the high bits are truncated, since any odd number is
  26 * relative prime to 2^n).
  27 *
  28 * Note also that the obfuscation itself is invisible to userspace and if needed
  29 * it can be changed to an alternate scheme.
  30 */
  31static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
  32
  33static long kptr_obfuscate(long v, int type)
  34{
  35        return (v ^ cookies[type][0]) * cookies[type][1];
  36}
  37
  38/*
  39 * 0 - equal, i.e. v1 = v2
  40 * 1 - less than, i.e. v1 < v2
  41 * 2 - greater than, i.e. v1 > v2
  42 * 3 - not equal but ordering unavailable (reserved for future)
  43 */
  44static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
  45{
  46        long ret;
  47
  48        ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);
  49
  50        return (ret < 0) | ((ret > 0) << 1);
  51}
  52
  53/* The caller must have pinned the task */
  54static struct file *
  55get_file_raw_ptr(struct task_struct *task, unsigned int idx)
  56{
  57        struct file *file = NULL;
  58
  59        task_lock(task);
  60        rcu_read_lock();
  61
  62        if (task->files)
  63                file = fcheck_files(task->files, idx);
  64
  65        rcu_read_unlock();
  66        task_unlock(task);
  67
  68        return file;
  69}
  70
  71static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
  72{
  73        if (likely(m2 != m1))
  74                mutex_unlock(m2);
  75        mutex_unlock(m1);
  76}
  77
  78static int kcmp_lock(struct mutex *m1, struct mutex *m2)
  79{
  80        int err;
  81
  82        if (m2 > m1)
  83                swap(m1, m2);
  84
  85        err = mutex_lock_killable(m1);
  86        if (!err && likely(m1 != m2)) {
  87                err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
  88                if (err)
  89                        mutex_unlock(m1);
  90        }
  91
  92        return err;
  93}
  94
  95SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
  96                unsigned long, idx1, unsigned long, idx2)
  97{
  98        struct task_struct *task1, *task2;
  99        int ret;
 100
 101        rcu_read_lock();
 102
 103        /*
 104         * Tasks are looked up in caller's PID namespace only.
 105         */
 106        task1 = find_task_by_vpid(pid1);
 107        task2 = find_task_by_vpid(pid2);
 108        if (!task1 || !task2)
 109                goto err_no_task;
 110
 111        get_task_struct(task1);
 112        get_task_struct(task2);
 113
 114        rcu_read_unlock();
 115
 116        /*
 117         * One should have enough rights to inspect task details.
 118         */
 119        ret = kcmp_lock(&task1->signal->cred_guard_mutex,
 120                        &task2->signal->cred_guard_mutex);
 121        if (ret)
 122                goto err;
 123        if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
 124            !ptrace_may_access(task2, PTRACE_MODE_READ)) {
 125                ret = -EPERM;
 126                goto err_unlock;
 127        }
 128
 129        switch (type) {
 130        case KCMP_FILE: {
 131                struct file *filp1, *filp2;
 132
 133                filp1 = get_file_raw_ptr(task1, idx1);
 134                filp2 = get_file_raw_ptr(task2, idx2);
 135
 136                if (filp1 && filp2)
 137                        ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
 138                else
 139                        ret = -EBADF;
 140                break;
 141        }
 142        case KCMP_VM:
 143                ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
 144                break;
 145        case KCMP_FILES:
 146                ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
 147                break;
 148        case KCMP_FS:
 149                ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
 150                break;
 151        case KCMP_SIGHAND:
 152                ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
 153                break;
 154        case KCMP_IO:
 155                ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
 156                break;
 157        case KCMP_SYSVSEM:
 158#ifdef CONFIG_SYSVIPC
 159                ret = kcmp_ptr(task1->sysvsem.undo_list,
 160                               task2->sysvsem.undo_list,
 161                               KCMP_SYSVSEM);
 162#else
 163                ret = -EOPNOTSUPP;
 164#endif
 165                break;
 166        default:
 167                ret = -EINVAL;
 168                break;
 169        }
 170
 171err_unlock:
 172        kcmp_unlock(&task1->signal->cred_guard_mutex,
 173                    &task2->signal->cred_guard_mutex);
 174err:
 175        put_task_struct(task1);
 176        put_task_struct(task2);
 177
 178        return ret;
 179
 180err_no_task:
 181        rcu_read_unlock();
 182        return -ESRCH;
 183}
 184
 185static __init int kcmp_cookies_init(void)
 186{
 187        int i;
 188
 189        get_random_bytes(cookies, sizeof(cookies));
 190
 191        for (i = 0; i < KCMP_TYPES; i++)
 192                cookies[i][1] |= (~(~0UL >>  1) | 1);
 193
 194        return 0;
 195}
 196arch_initcall(kcmp_cookies_init);
 197
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