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 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 */
  55 static struct file *
  56 get_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
  72 static 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
  79 static 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
  96 SYSCALL_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
 172 err_unlock:
 173         kcmp_unlock(&task1->signal->cred_guard_mutex,
 174                     &task2->signal->cred_guard_mutex);
 175 err:
 176         put_task_struct(task1);
 177         put_task_struct(task2);
 178
 179         return ret;
 180
 181 err_no_task:
 182         rcu_read_unlock();
 183         return -ESRCH;
 184 }
 185
 186 static __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 }
 197 arch_initcall(kcmp_cookies_init);