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 #include <linux/capability.h>
  15 #include <linux/list.h>
  16 #include <linux/eventpoll.h>
  17 #include <linux/file.h>
  18
  19 #include <asm/unistd.h>
  20
  21 /*
  22  * We don't expose the real in-memory order of objects for security reasons.
  23  * But still the comparison results should be suitable for sorting. So we
  24  * obfuscate kernel pointers values and compare the production instead.
  25  *
  26  * The obfuscation is done in two steps. First we xor the kernel pointer with
  27  * a random value, which puts pointer into a new position in a reordered space.
  28  * Secondly we multiply the xor production with a large odd random number to
  29  * permute its bits even more (the odd multiplier guarantees that the product
  30  * is unique ever after the high bits are truncated, since any odd number is
  31  * relative prime to 2^n).
  32  *
  33  * Note also that the obfuscation itself is invisible to userspace and if needed
  34  * it can be changed to an alternate scheme.
  35  */
  36 static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
  37
  38 static long kptr_obfuscate(long v, int type)
  39 {
  40         return (v ^ cookies[type][0]) * cookies[type][1];
  41 }
  42
  43 /*
  44  * 0 - equal, i.e. v1 = v2
  45  * 1 - less than, i.e. v1 < v2
  46  * 2 - greater than, i.e. v1 > v2
  47  * 3 - not equal but ordering unavailable (reserved for future)
  48  */
  49 static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
  50 {
  51         long t1, t2;
  52
  53         t1 = kptr_obfuscate((long)v1, type);
  54         t2 = kptr_obfuscate((long)v2, type);
  55
  56         return (t1 < t2) | ((t1 > t2) << 1);
  57 }
  58
  59 /* The caller must have pinned the task */
  60 static struct file *
  61 get_file_raw_ptr(struct task_struct *task, unsigned int idx)
  62 {
  63         struct file *file = NULL;
  64
  65         task_lock(task);
  66         rcu_read_lock();
  67
  68         if (task->files)
  69                 file = fcheck_files(task->files, idx);
  70
  71         rcu_read_unlock();
  72         task_unlock(task);
  73
  74         return file;
  75 }
  76
  77 static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
  78 {
  79         if (likely(m2 != m1))
  80                 mutex_unlock(m2);
  81         mutex_unlock(m1);
  82 }
  83
  84 static int kcmp_lock(struct mutex *m1, struct mutex *m2)
  85 {
  86         int err;
  87
  88         if (m2 > m1)
  89                 swap(m1, m2);
  90
  91         err = mutex_lock_killable(m1);
  92         if (!err && likely(m1 != m2)) {
  93                 err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
  94                 if (err)
  95                         mutex_unlock(m1);
  96         }
  97
  98         return err;
  99 }
 100
 101 #ifdef CONFIG_EPOLL
 102 static int kcmp_epoll_target(struct task_struct *task1,
 103                              struct task_struct *task2,
 104                              unsigned long idx1,
 105                              struct kcmp_epoll_slot __user *uslot)
 106 {
 107         struct file *filp, *filp_epoll, *filp_tgt;
 108         struct kcmp_epoll_slot slot;
 109         struct files_struct *files;
 110
 111         if (copy_from_user(&slot, uslot, sizeof(slot)))
 112                 return -EFAULT;
 113
 114         filp = get_file_raw_ptr(task1, idx1);
 115         if (!filp)
 116                 return -EBADF;
 117
 118         files = get_files_struct(task2);
 119         if (!files)
 120                 return -EBADF;
 121
 122         spin_lock(&files->file_lock);
 123         filp_epoll = fcheck_files(files, slot.efd);
 124         if (filp_epoll)
 125                 get_file(filp_epoll);
 126         else
 127                 filp_tgt = ERR_PTR(-EBADF);
 128         spin_unlock(&files->file_lock);
 129         put_files_struct(files);
 130
 131         if (filp_epoll) {
 132                 filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
 133                 fput(filp_epoll);
 134         }
 135
 136         if (IS_ERR(filp_tgt))
 137                 return PTR_ERR(filp_tgt);
 138
 139         return kcmp_ptr(filp, filp_tgt, KCMP_FILE);
 140 }
 141 #else
 142 static int kcmp_epoll_target(struct task_struct *task1,
 143                              struct task_struct *task2,
 144                              unsigned long idx1,
 145                              struct kcmp_epoll_slot __user *uslot)
 146 {
 147         return -EOPNOTSUPP;
 148 }
 149 #endif
 150
 151 SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
 152                 unsigned long, idx1, unsigned long, idx2)
 153 {
 154         struct task_struct *task1, *task2;
 155         int ret;
 156
 157         rcu_read_lock();
 158
 159         /*
 160          * Tasks are looked up in caller's PID namespace only.
 161          */
 162         task1 = find_task_by_vpid(pid1);
 163         task2 = find_task_by_vpid(pid2);
 164         if (!task1 || !task2)
 165                 goto err_no_task;
 166
 167         get_task_struct(task1);
 168         get_task_struct(task2);
 169
 170         rcu_read_unlock();
 171
 172         /*
 173          * One should have enough rights to inspect task details.
 174          */
 175         ret = kcmp_lock(&task1->signal->cred_guard_mutex,
 176                         &task2->signal->cred_guard_mutex);
 177         if (ret)
 178                 goto err;
 179         if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
 180             !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
 181                 ret = -EPERM;
 182                 goto err_unlock;
 183         }
 184
 185         switch (type) {
 186         case KCMP_FILE: {
 187                 struct file *filp1, *filp2;
 188
 189                 filp1 = get_file_raw_ptr(task1, idx1);
 190                 filp2 = get_file_raw_ptr(task2, idx2);
 191
 192                 if (filp1 && filp2)
 193                         ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
 194                 else
 195                         ret = -EBADF;
 196                 break;
 197         }
 198         case KCMP_VM:
 199                 ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
 200                 break;
 201         case KCMP_FILES:
 202                 ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
 203                 break;
 204         case KCMP_FS:
 205                 ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
 206                 break;
 207         case KCMP_SIGHAND:
 208                 ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
 209                 break;
 210         case KCMP_IO:
 211                 ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
 212                 break;
 213         case KCMP_SYSVSEM:
 214 #ifdef CONFIG_SYSVIPC
 215                 ret = kcmp_ptr(task1->sysvsem.undo_list,
 216                                task2->sysvsem.undo_list,
 217                                KCMP_SYSVSEM);
 218 #else
 219                 ret = -EOPNOTSUPP;
 220 #endif
 221                 break;
 222         case KCMP_EPOLL_TFD:
 223                 ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
 224                 break;
 225         default:
 226                 ret = -EINVAL;
 227                 break;
 228         }
 229
 230 err_unlock:
 231         kcmp_unlock(&task1->signal->cred_guard_mutex,
 232                     &task2->signal->cred_guard_mutex);
 233 err:
 234         put_task_struct(task1);
 235         put_task_struct(task2);
 236
 237         return ret;
 238
 239 err_no_task:
 240         rcu_read_unlock();
 241         return -ESRCH;
 242 }
 243
 244 static __init int kcmp_cookies_init(void)
 245 {
 246         int i;
 247
 248         get_random_bytes(cookies, sizeof(cookies));
 249
 250         for (i = 0; i < KCMP_TYPES; i++)
 251                 cookies[i][1] |= (~(~0UL >>  1) | 1);
 252
 253         return 0;
 254 }
 255 arch_initcall(kcmp_cookies_init);