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

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