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  */
  32 static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
  33
  34 static 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  */
  45 static 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 */
  56 static struct file *
  57 get_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
  73 static 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
  80 static 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
  97 SYSCALL_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) ||
 126             !ptrace_may_access(task2, PTRACE_MODE_READ)) {
 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
 173 err_unlock:
 174         kcmp_unlock(&task1->signal->cred_guard_mutex,
 175                     &task2->signal->cred_guard_mutex);
 176 err:
 177         put_task_struct(task1);
 178         put_task_struct(task2);
 179
 180         return ret;
 181
 182 err_no_task:
 183         rcu_read_unlock();
 184         return -ESRCH;
 185 }
 186
 187 static __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 }
 198 arch_initcall(kcmp_cookies_init);