LXR linux/kernel/kcmp.c

   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 t1, t2;
  48
  49        t1 = kptr_obfuscate((long)v1, type);
  50        t2 = kptr_obfuscate((long)v2, type);
  51
  52        return (t1 < t2) | ((t1 > t2) << 1);
  53}
  54
  55/* The caller must have pinned the task */
  56static struct file *
  57get_file_raw_ptr(struct task_struct *task, unsigned int idx)
  58{
  59        struct file *file = NULL;
  60
  61        task_lock(task);
  62        rcu_read_lock();
  63
  64        if (task->files)
  65                file = fcheck_files(task->files, idx);
  66
  67        rcu_read_unlock();
  68        task_unlock(task);
  69
  70        return file;
  71}
  72
  73static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
  74{
  75        if (likely(m2 != m1))
  76                mutex_unlock(m2);
  77        mutex_unlock(m1);
  78}
  79
  80static int kcmp_lock(struct mutex *m1, struct mutex *m2)
  81{
  82        int err;
  83
  84        if (m2 > m1)
  85                swap(m1, m2);
  86
  87        err = mutex_lock_killable(m1);
  88        if (!err && likely(m1 != m2)) {
  89                err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
  90                if (err)
  91                        mutex_unlock(m1);
  92        }
  93
  94        return err;
  95}
  96
  97SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
  98                unsigned long, idx1, unsigned long, idx2)
  99{
 100        struct task_struct *task1, *task2;
 101        int ret;
 102
 103        rcu_read_lock();
 104
 105        /*
 106         * Tasks are looked up in caller's PID namespace only.
 107         */
 108        task1 = find_task_by_vpid(pid1);
 109        task2 = find_task_by_vpid(pid2);
 110        if (!task1 || !task2)
 111                goto err_no_task;
 112
 113        get_task_struct(task1);
 114        get_task_struct(task2);
 115
 116        rcu_read_unlock();
 117
 118        /*
 119         * One should have enough rights to inspect task details.
 120         */
 121        ret = kcmp_lock(&task1->signal->cred_guard_mutex,
 122                        &task2->signal->cred_guard_mutex);
 123        if (ret)
 124                goto err;
 125        if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
 126            !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
 127                ret = -EPERM;
 128                goto err_unlock;
 129        }
 130
 131        switch (type) {
 132        case KCMP_FILE: {
 133                struct file *filp1, *filp2;
 134
 135                filp1 = get_file_raw_ptr(task1, idx1);
 136                filp2 = get_file_raw_ptr(task2, idx2);
 137
 138                if (filp1 && filp2)
 139                        ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
 140                else
 141                        ret = -EBADF;
 142                break;
 143        }
 144        case KCMP_VM:
 145                ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
 146                break;
 147        case KCMP_FILES:
 148                ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
 149                break;
 150        case KCMP_FS:
 151                ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
 152                break;
 153        case KCMP_SIGHAND:
 154                ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
 155                break;
 156        case KCMP_IO:
 157                ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
 158                break;
 159        case KCMP_SYSVSEM:
 160#ifdef CONFIG_SYSVIPC
 161                ret = kcmp_ptr(task1->sysvsem.undo_list,
 162                               task2->sysvsem.undo_list,
 163                               KCMP_SYSVSEM);
 164#else
 165                ret = -EOPNOTSUPP;
 166#endif
 167                break;
 168        default:
 169                ret = -EINVAL;
 170                break;
 171        }
 172
 173err_unlock:
 174        kcmp_unlock(&task1->signal->cred_guard_mutex,
 175                    &task2->signal->cred_guard_mutex);
 176err:
 177        put_task_struct(task1);
 178        put_task_struct(task2);
 179
 180        return ret;
 181
 182err_no_task:
 183        rcu_read_unlock();
 184        return -ESRCH;
 185}
 186
 187static __init int kcmp_cookies_init(void)
 188{
 189        int i;
 190
 191        get_random_bytes(cookies, sizeof(cookies));
 192
 193        for (i = 0; i < KCMP_TYPES; i++)
 194                cookies[i][1] |= (~(~0UL >>  1) | 1);
 195
 196        return 0;
 197}
 198arch_initcall(kcmp_cookies_init);
 199