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/ptrace.h>
   8#include <linux/init.h>
   9#include <linux/errno.h>
  10#include <linux/cache.h>
  11#include <linux/bug.h>
  12#include <linux/err.h>
  13#include <linux/kcmp.h>
  14
  15#include <asm/unistd.h>
  16
  17/*
  18 * We don't expose the real in-memory order of objects for security reasons.
  19 * But still the comparison results should be suitable for sorting. So we
  20 * obfuscate kernel pointers values and compare the production instead.
  21 *
  22 * The obfuscation is done in two steps. First we xor the kernel pointer with
  23 * a random value, which puts pointer into a new position in a reordered space.
  24 * Secondly we multiply the xor production with a large odd random number to
  25 * permute its bits even more (the odd multiplier guarantees that the product
  26 * is unique ever after the high bits are truncated, since any odd number is
  27 * relative prime to 2^n).
  28 *
  29 * Note also that the obfuscation itself is invisible to userspace and if needed
  30 * it can be changed to an alternate scheme.
  31 */
  32static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
  33
  34static long kptr_obfuscate(long v, int type)
  35{
  36        return (v ^ cookies[type][0]) * cookies[type][1];
  37}
  38
  39/*
  40 * 0 - equal, i.e. v1 = v2
  41 * 1 - less than, i.e. v1 < v2
  42 * 2 - greater than, i.e. v1 > v2
  43 * 3 - not equal but ordering unavailable (reserved for future)
  44 */
  45static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
  46{
  47        long ret;
  48
  49        ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);
  50
  51        return (ret < 0) | ((ret > 0) << 1);
  52}
  53
  54/* The caller must have pinned the task */
  55static struct file *
  56get_file_raw_ptr(struct task_struct *task, unsigned int idx)
  57{
  58        struct file *file = NULL;
  59
  60        task_lock(task);
  61        rcu_read_lock();
  62
  63        if (task->files)
  64                file = fcheck_files(task->files, idx);
  65
  66        rcu_read_unlock();
  67        task_unlock(task);
  68
  69        return file;
  70}
  71
  72static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
  73{
  74        if (likely(m2 != m1))
  75                mutex_unlock(m2);
  76        mutex_unlock(m1);
  77}
  78
  79static int kcmp_lock(struct mutex *m1, struct mutex *m2)
  80{
  81        int err;
  82
  83        if (m2 > m1)
  84                swap(m1, m2);
  85
  86        err = mutex_lock_killable(m1);
  87        if (!err && likely(m1 != m2)) {
  88                err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
  89                if (err)
  90                        mutex_unlock(m1);
  91        }
  92
  93        return err;
  94}
  95
  96SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
  97                unsigned long, idx1, unsigned long, idx2)
  98{
  99        struct task_struct *task1, *task2;
 100        int ret;
 101
 102        rcu_read_lock();
 103
 104        /*
 105         * Tasks are looked up in caller's PID namespace only.
 106         */
 107        task1 = find_task_by_vpid(pid1);
 108        task2 = find_task_by_vpid(pid2);
 109        if (!task1 || !task2)
 110                goto err_no_task;
 111
 112        get_task_struct(task1);
 113        get_task_struct(task2);
 114
 115        rcu_read_unlock();
 116
 117        /*
 118         * One should have enough rights to inspect task details.
 119         */
 120        ret = kcmp_lock(&task1->signal->cred_guard_mutex,
 121                        &task2->signal->cred_guard_mutex);
 122        if (ret)
 123                goto err;
 124        if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
 125            !ptrace_may_access(task2, PTRACE_MODE_READ)) {
 126                ret = -EPERM;
 127                goto err_unlock;
 128        }
 129
 130        switch (type) {
 131        case KCMP_FILE: {
 132                struct file *filp1, *filp2;
 133
 134                filp1 = get_file_raw_ptr(task1, idx1);
 135                filp2 = get_file_raw_ptr(task2, idx2);
 136
 137                if (filp1 && filp2)
 138                        ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
 139                else
 140                        ret = -EBADF;
 141                break;
 142        }
 143        case KCMP_VM:
 144                ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
 145                break;
 146        case KCMP_FILES:
 147                ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
 148                break;
 149        case KCMP_FS:
 150                ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
 151                break;
 152        case KCMP_SIGHAND:
 153                ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
 154                break;
 155        case KCMP_IO:
 156                ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
 157                break;
 158        case KCMP_SYSVSEM:
 159#ifdef CONFIG_SYSVIPC
 160                ret = kcmp_ptr(task1->sysvsem.undo_list,
 161                               task2->sysvsem.undo_list,
 162                               KCMP_SYSVSEM);
 163#else
 164                ret = -EOPNOTSUPP;
 165#endif
 166                break;
 167        default:
 168                ret = -EINVAL;
 169                break;
 170        }
 171
 172err_unlock:
 173        kcmp_unlock(&task1->signal->cred_guard_mutex,
 174                    &task2->signal->cred_guard_mutex);
 175err:
 176        put_task_struct(task1);
 177        put_task_struct(task2);
 178
 179        return ret;
 180
 181err_no_task:
 182        rcu_read_unlock();
 183        return -ESRCH;
 184}
 185
 186static __init int kcmp_cookies_init(void)
 187{
 188        int i;
 189
 190        get_random_bytes(cookies, sizeof(cookies));
 191
 192        for (i = 0; i < KCMP_TYPES; i++)
 193                cookies[i][1] |= (~(~0UL >>  1) | 1);
 194
 195        return 0;
 196}
 197arch_initcall(kcmp_cookies_init);
 198
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