kernel/kcmp.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/kernel.h>
   3 #include <linux/syscalls.h>
   4 #include <linux/fdtable.h>
   5 #include <linux/string.h>
   6 #include <linux/random.h>
   7 #include <linux/module.h>
   8 #include <linux/ptrace.h>
   9 #include <linux/init.h>
  10 #include <linux/errno.h>
  11 #include <linux/cache.h>
  12 #include <linux/bug.h>
  13 #include <linux/err.h>
  14 #include <linux/kcmp.h>
  15 #include <linux/capability.h>
  16 #include <linux/list.h>
  17 #include <linux/eventpoll.h>
  18 #include <linux/file.h>
  19
  20 #include <asm/unistd.h>
  21
  22 /*
  23  * We don't expose the real in-memory order of objects for security reasons.
  24  * But still the comparison results should be suitable for sorting. So we
  25  * obfuscate kernel pointers values and compare the production instead.
  26  *
  27  * The obfuscation is done in two steps. First we xor the kernel pointer with
  28  * a random value, which puts pointer into a new position in a reordered space.
  29  * Secondly we multiply the xor production with a large odd random number to
  30  * permute its bits even more (the odd multiplier guarantees that the product
  31  * is unique ever after the high bits are truncated, since any odd number is
  32  * relative prime to 2^n).
  33  *
  34  * Note also that the obfuscation itself is invisible to userspace and if needed
  35  * it can be changed to an alternate scheme.
  36  */
  37 static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
  38
  39 static long kptr_obfuscate(long v, int type)
  40 {
  41         return (v ^ cookies[type][0]) * cookies[type][1];
  42 }
  43
  44 /*
  45  * 0 - equal, i.e. v1 = v2
  46  * 1 - less than, i.e. v1 < v2
  47  * 2 - greater than, i.e. v1 > v2
  48  * 3 - not equal but ordering unavailable (reserved for future)
  49  */
  50 static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
  51 {
  52         long t1, t2;
  53
  54         t1 = kptr_obfuscate((long)v1, type);
  55         t2 = kptr_obfuscate((long)v2, type);
  56
  57         return (t1 < t2) | ((t1 > t2) << 1);
  58 }
  59
  60 /* The caller must have pinned the task */
  61 static struct file *
  62 get_file_raw_ptr(struct task_struct *task, unsigned int idx)
  63 {
  64         struct file *file;
  65
  66         file = fget_task(task, idx);
  67         if (file)
  68                 fput(file);
  69
  70         return file;
  71 }
  72
  73 static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
  74 {
  75         if (likely(l2 != l1))
  76                 up_read(l2);
  77         up_read(l1);
  78 }
  79
  80 static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
  81 {
  82         int err;
  83
  84         if (l2 > l1)
  85                 swap(l1, l2);
  86
  87         err = down_read_killable(l1);
  88         if (!err && likely(l1 != l2)) {
  89                 err = down_read_killable_nested(l2, SINGLE_DEPTH_NESTING);
  90                 if (err)
  91                         up_read(l1);
  92         }
  93
  94         return err;
  95 }
  96
  97 #ifdef CONFIG_EPOLL
  98 static int kcmp_epoll_target(struct task_struct *task1,
  99                              struct task_struct *task2,
 100                              unsigned long idx1,
 101                              struct kcmp_epoll_slot __user *uslot)
 102 {
 103         struct file *filp, *filp_epoll, *filp_tgt;
 104         struct kcmp_epoll_slot slot;
 105
 106         if (copy_from_user(&slot, uslot, sizeof(slot)))
 107                 return -EFAULT;
 108
 109         filp = get_file_raw_ptr(task1, idx1);
 110         if (!filp)
 111                 return -EBADF;
 112
 113         filp_epoll = fget_task(task2, slot.efd);
 114         if (!filp_epoll)
 115                 return -EBADF;
 116
 117         filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
 118         fput(filp_epoll);
 119
 120         if (IS_ERR(filp_tgt))
 121                 return PTR_ERR(filp_tgt);
 122
 123         return kcmp_ptr(filp, filp_tgt, KCMP_FILE);
 124 }
 125 #else
 126 static int kcmp_epoll_target(struct task_struct *task1,
 127                              struct task_struct *task2,
 128                              unsigned long idx1,
 129                              struct kcmp_epoll_slot __user *uslot)
 130 {
 131         return -EOPNOTSUPP;
 132 }
 133 #endif
 134
 135 SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
 136                 unsigned long, idx1, unsigned long, idx2)
 137 {
 138         struct task_struct *task1, *task2;
 139         int ret;
 140
 141         rcu_read_lock();
 142
 143         /*
 144          * Tasks are looked up in caller's PID namespace only.
 145          */
 146         task1 = find_task_by_vpid(pid1);
 147         task2 = find_task_by_vpid(pid2);
 148         if (!task1 || !task2)
 149                 goto err_no_task;
 150
 151         get_task_struct(task1);
 152         get_task_struct(task2);
 153
 154         rcu_read_unlock();
 155
 156         /*
 157          * One should have enough rights to inspect task details.
 158          */
 159         ret = kcmp_lock(&task1->signal->exec_update_lock,
 160                         &task2->signal->exec_update_lock);
 161         if (ret)
 162                 goto err;
 163         if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
 164             !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
 165                 ret = -EPERM;
 166                 goto err_unlock;
 167         }
 168
 169         switch (type) {
 170         case KCMP_FILE: {
 171                 struct file *filp1, *filp2;
 172
 173                 filp1 = get_file_raw_ptr(task1, idx1);
 174                 filp2 = get_file_raw_ptr(task2, idx2);
 175
 176                 if (filp1 && filp2)
 177                         ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
 178                 else
 179                         ret = -EBADF;
 180                 break;
 181         }
 182         case KCMP_VM:
 183                 ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
 184                 break;
 185         case KCMP_FILES:
 186                 ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
 187                 break;
 188         case KCMP_FS:
 189                 ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
 190                 break;
 191         case KCMP_SIGHAND:
 192                 ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
 193                 break;
 194         case KCMP_IO:
 195                 ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
 196                 break;
 197         case KCMP_SYSVSEM:
 198 #ifdef CONFIG_SYSVIPC
 199                 ret = kcmp_ptr(task1->sysvsem.undo_list,
 200                                task2->sysvsem.undo_list,
 201                                KCMP_SYSVSEM);
 202 #else
 203                 ret = -EOPNOTSUPP;
 204 #endif
 205                 break;
 206         case KCMP_EPOLL_TFD:
 207                 ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
 208                 break;
 209         default:
 210                 ret = -EINVAL;
 211                 break;
 212         }
 213
 214 err_unlock:
 215         kcmp_unlock(&task1->signal->exec_update_lock,
 216                     &task2->signal->exec_update_lock);
 217 err:
 218         put_task_struct(task1);
 219         put_task_struct(task2);
 220
 221         return ret;
 222
 223 err_no_task:
 224         rcu_read_unlock();
 225         return -ESRCH;
 226 }
 227
 228 static __init int kcmp_cookies_init(void)
 229 {
 230         int i;
 231
 232         get_random_bytes(cookies, sizeof(cookies));
 233
 234         for (i = 0; i < KCMP_TYPES; i++)
 235                 cookies[i][1] |= (~(~0UL >>  1) | 1);
 236
 237         return 0;
 238 }
 239 arch_initcall(kcmp_cookies_init);