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/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  */
  31 static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
  32
  33 static 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  */
  44 static 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 */
  54 static struct file *
  55 get_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
  71 static 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
  78 static 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
  95 SYSCALL_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
 171 err_unlock:
 172         kcmp_unlock(&task1->signal->cred_guard_mutex,
 173                     &task2->signal->cred_guard_mutex);
 174 err:
 175         put_task_struct(task1);
 176         put_task_struct(task2);
 177
 178         return ret;
 179
 180 err_no_task:
 181         rcu_read_unlock();
 182         return -ESRCH;
 183 }
 184
 185 static __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 }
 196 arch_initcall(kcmp_cookies_init);