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         rcu_read_lock();
  67         file = task_lookup_fdget_rcu(task, idx);
  68         rcu_read_unlock();
  69         if (file)
  70                 fput(file);
  71
  72         return file;
  73 }
  74
  75 static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
  76 {
  77         if (likely(l2 != l1))
  78                 up_read(l2);
  79         up_read(l1);
  80 }
  81
  82 static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
  83 {
  84         int err;
  85
  86         if (l2 > l1)
  87                 swap(l1, l2);
  88
  89         err = down_read_killable(l1);
  90         if (!err && likely(l1 != l2)) {
  91                 err = down_read_killable_nested(l2, SINGLE_DEPTH_NESTING);
  92                 if (err)
  93                         up_read(l1);
  94         }
  95
  96         return err;
  97 }
  98
  99 #ifdef CONFIG_EPOLL
 100 static int kcmp_epoll_target(struct task_struct *task1,
 101                              struct task_struct *task2,
 102                              unsigned long idx1,
 103                              struct kcmp_epoll_slot __user *uslot)
 104 {
 105         struct file *filp, *filp_epoll, *filp_tgt;
 106         struct kcmp_epoll_slot slot;
 107
 108         if (copy_from_user(&slot, uslot, sizeof(slot)))
 109                 return -EFAULT;
 110
 111         filp = get_file_raw_ptr(task1, idx1);
 112         if (!filp)
 113                 return -EBADF;
 114
 115         filp_epoll = fget_task(task2, slot.efd);
 116         if (!filp_epoll)
 117                 return -EBADF;
 118
 119         filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
 120         fput(filp_epoll);
 121
 122         if (IS_ERR(filp_tgt))
 123                 return PTR_ERR(filp_tgt);
 124
 125         return kcmp_ptr(filp, filp_tgt, KCMP_FILE);
 126 }
 127 #else
 128 static int kcmp_epoll_target(struct task_struct *task1,
 129                              struct task_struct *task2,
 130                              unsigned long idx1,
 131                              struct kcmp_epoll_slot __user *uslot)
 132 {
 133         return -EOPNOTSUPP;
 134 }
 135 #endif
 136
 137 SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
 138                 unsigned long, idx1, unsigned long, idx2)
 139 {
 140         struct task_struct *task1, *task2;
 141         int ret;
 142
 143         rcu_read_lock();
 144
 145         /*
 146          * Tasks are looked up in caller's PID namespace only.
 147          */
 148         task1 = find_task_by_vpid(pid1);
 149         task2 = find_task_by_vpid(pid2);
 150         if (!task1 || !task2)
 151                 goto err_no_task;
 152
 153         get_task_struct(task1);
 154         get_task_struct(task2);
 155
 156         rcu_read_unlock();
 157
 158         /*
 159          * One should have enough rights to inspect task details.
 160          */
 161         ret = kcmp_lock(&task1->signal->exec_update_lock,
 162                         &task2->signal->exec_update_lock);
 163         if (ret)
 164                 goto err;
 165         if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
 166             !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
 167                 ret = -EPERM;
 168                 goto err_unlock;
 169         }
 170
 171         switch (type) {
 172         case KCMP_FILE: {
 173                 struct file *filp1, *filp2;
 174
 175                 filp1 = get_file_raw_ptr(task1, idx1);
 176                 filp2 = get_file_raw_ptr(task2, idx2);
 177
 178                 if (filp1 && filp2)
 179                         ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
 180                 else
 181                         ret = -EBADF;
 182                 break;
 183         }
 184         case KCMP_VM:
 185                 ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
 186                 break;
 187         case KCMP_FILES:
 188                 ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
 189                 break;
 190         case KCMP_FS:
 191                 ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
 192                 break;
 193         case KCMP_SIGHAND:
 194                 ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
 195                 break;
 196         case KCMP_IO:
 197                 ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
 198                 break;
 199         case KCMP_SYSVSEM:
 200 #ifdef CONFIG_SYSVIPC
 201                 ret = kcmp_ptr(task1->sysvsem.undo_list,
 202                                task2->sysvsem.undo_list,
 203                                KCMP_SYSVSEM);
 204 #else
 205                 ret = -EOPNOTSUPP;
 206 #endif
 207                 break;
 208         case KCMP_EPOLL_TFD:
 209                 ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
 210                 break;
 211         default:
 212                 ret = -EINVAL;
 213                 break;
 214         }
 215
 216 err_unlock:
 217         kcmp_unlock(&task1->signal->exec_update_lock,
 218                     &task2->signal->exec_update_lock);
 219 err:
 220         put_task_struct(task1);
 221         put_task_struct(task2);
 222
 223         return ret;
 224
 225 err_no_task:
 226         rcu_read_unlock();
 227         return -ESRCH;
 228 }
 229
 230 static __init int kcmp_cookies_init(void)
 231 {
 232         int i;
 233
 234         get_random_bytes(cookies, sizeof(cookies));
 235
 236         for (i = 0; i < KCMP_TYPES; i++)
 237                 cookies[i][1] |= (~(~0UL >>  1) | 1);
 238
 239         return 0;
 240 }
 241 arch_initcall(kcmp_cookies_init);