root/kernel/kcmp.c

DEFINITIONS

1. kptr_obfuscate
2. kcmp_ptr
3. get_file_raw_ptr
4. kcmp_unlock
5. kcmp_lock
6. kcmp_epoll_target
7. kcmp_epoll_target
8. SYSCALL_DEFINE5
9. kcmp_cookies_init

   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 = NULL;
  65 
  66         task_lock(task);
  67         rcu_read_lock();
  68 
  69         if (task->files)
  70                 file = fcheck_files(task->files, idx);
  71 
  72         rcu_read_unlock();
  73         task_unlock(task);
  74 
  75         return file;
  76 }
  77 
  78 static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
  79 {
  80         if (likely(m2 != m1))
  81                 mutex_unlock(m2);
  82         mutex_unlock(m1);
  83 }
  84 
  85 static int kcmp_lock(struct mutex *m1, struct mutex *m2)
  86 {
  87         int err;
  88 
  89         if (m2 > m1)
  90                 swap(m1, m2);
  91 
  92         err = mutex_lock_killable(m1);
  93         if (!err && likely(m1 != m2)) {
  94                 err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
  95                 if (err)
  96                         mutex_unlock(m1);
  97         }
  98 
  99         return err;
 100 }
 101 
 102 #ifdef CONFIG_EPOLL
 103 static int kcmp_epoll_target(struct task_struct *task1,
 104                              struct task_struct *task2,
 105                              unsigned long idx1,
 106                              struct kcmp_epoll_slot __user *uslot)
 107 {
 108         struct file *filp, *filp_epoll, *filp_tgt;
 109         struct kcmp_epoll_slot slot;
 110         struct files_struct *files;
 111 
 112         if (copy_from_user(&slot, uslot, sizeof(slot)))
 113                 return -EFAULT;
 114 
 115         filp = get_file_raw_ptr(task1, idx1);
 116         if (!filp)
 117                 return -EBADF;
 118 
 119         files = get_files_struct(task2);
 120         if (!files)
 121                 return -EBADF;
 122 
 123         spin_lock(&files->file_lock);
 124         filp_epoll = fcheck_files(files, slot.efd);
 125         if (filp_epoll)
 126                 get_file(filp_epoll);
 127         else
 128                 filp_tgt = ERR_PTR(-EBADF);
 129         spin_unlock(&files->file_lock);
 130         put_files_struct(files);
 131 
 132         if (filp_epoll) {
 133                 filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
 134                 fput(filp_epoll);
 135         }
 136 
 137         if (IS_ERR(filp_tgt))
 138                 return PTR_ERR(filp_tgt);
 139 
 140         return kcmp_ptr(filp, filp_tgt, KCMP_FILE);
 141 }
 142 #else
 143 static int kcmp_epoll_target(struct task_struct *task1,
 144                              struct task_struct *task2,
 145                              unsigned long idx1,
 146                              struct kcmp_epoll_slot __user *uslot)
 147 {
 148         return -EOPNOTSUPP;
 149 }
 150 #endif
 151 
 152 SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
 153                 unsigned long, idx1, unsigned long, idx2)
 154 {
 155         struct task_struct *task1, *task2;
 156         int ret;
 157 
 158         rcu_read_lock();
 159 
 160         /*
 161          * Tasks are looked up in caller's PID namespace only.
 162          */
 163         task1 = find_task_by_vpid(pid1);
 164         task2 = find_task_by_vpid(pid2);
 165         if (!task1 || !task2)
 166                 goto err_no_task;
 167 
 168         get_task_struct(task1);
 169         get_task_struct(task2);
 170 
 171         rcu_read_unlock();
 172 
 173         /*
 174          * One should have enough rights to inspect task details.
 175          */
 176         ret = kcmp_lock(&task1->signal->cred_guard_mutex,
 177                         &task2->signal->cred_guard_mutex);
 178         if (ret)
 179                 goto err;
 180         if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
 181             !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
 182                 ret = -EPERM;
 183                 goto err_unlock;
 184         }
 185 
 186         switch (type) {
 187         case KCMP_FILE: {
 188                 struct file *filp1, *filp2;
 189 
 190                 filp1 = get_file_raw_ptr(task1, idx1);
 191                 filp2 = get_file_raw_ptr(task2, idx2);
 192 
 193                 if (filp1 && filp2)
 194                         ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
 195                 else
 196                         ret = -EBADF;
 197                 break;
 198         }
 199         case KCMP_VM:
 200                 ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
 201                 break;
 202         case KCMP_FILES:
 203                 ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
 204                 break;
 205         case KCMP_FS:
 206                 ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
 207                 break;
 208         case KCMP_SIGHAND:
 209                 ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
 210                 break;
 211         case KCMP_IO:
 212                 ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
 213                 break;
 214         case KCMP_SYSVSEM:
 215 #ifdef CONFIG_SYSVIPC
 216                 ret = kcmp_ptr(task1->sysvsem.undo_list,
 217                                task2->sysvsem.undo_list,
 218                                KCMP_SYSVSEM);
 219 #else
 220                 ret = -EOPNOTSUPP;
 221 #endif
 222                 break;
 223         case KCMP_EPOLL_TFD:
 224                 ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
 225                 break;
 226         default:
 227                 ret = -EINVAL;
 228                 break;
 229         }
 230 
 231 err_unlock:
 232         kcmp_unlock(&task1->signal->cred_guard_mutex,
 233                     &task2->signal->cred_guard_mutex);
 234 err:
 235         put_task_struct(task1);
 236         put_task_struct(task2);
 237 
 238         return ret;
 239 
 240 err_no_task:
 241         rcu_read_unlock();
 242         return -ESRCH;
 243 }
 244 
 245 static __init int kcmp_cookies_init(void)
 246 {
 247         int i;
 248 
 249         get_random_bytes(cookies, sizeof(cookies));
 250 
 251         for (i = 0; i < KCMP_TYPES; i++)
 252                 cookies[i][1] |= (~(~0UL >>  1) | 1);
 253 
 254         return 0;
 255 }
 256 arch_initcall(kcmp_cookies_init);