1 files changed, 196 insertions, 0 deletions

diff --git a/kernel/kcmp.c b/kernel/kcmp.c
new file mode 100644
index 000000000000..30b7b225306c
--- /dev/null
+++ b/kernel/kcmp.c
@@ -0,0 +1,196 @@
+#include <linux/kernel.h>
+#include <linux/syscalls.h>
+#include <linux/fdtable.h>
+#include <linux/string.h>
+#include <linux/random.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/errno.h>
+#include <linux/cache.h>
+#include <linux/bug.h>
+#include <linux/err.h>
+#include <linux/kcmp.h>
+
+#include <asm/unistd.h>
+
+/*
+ * We don't expose the real in-memory order of objects for security reasons.
+ * But still the comparison results should be suitable for sorting. So we
+ * obfuscate kernel pointers values and compare the production instead.
+ *
+ * The obfuscation is done in two steps. First we xor the kernel pointer with
+ * a random value, which puts pointer into a new position in a reordered space.
+ * Secondly we multiply the xor production with a large odd random number to
+ * permute its bits even more (the odd multiplier guarantees that the product
+ * is unique ever after the high bits are truncated, since any odd number is
+ * relative prime to 2^n).
+ *
+ * Note also that the obfuscation itself is invisible to userspace and if needed
+ * it can be changed to an alternate scheme.
+ */
+static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
+
+static long kptr_obfuscate(long v, int type)
+{
+        return (v ^ cookies[type][0]) * cookies[type][1];
+}
+
+/*
+ * 0 - equal, i.e. v1 = v2
+ * 1 - less than, i.e. v1 < v2
+ * 2 - greater than, i.e. v1 > v2
+ * 3 - not equal but ordering unavailable (reserved for future)
+ */
+static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
+{
+        long ret;
+
+        ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);
+
+        return (ret < 0) | ((ret > 0) << 1);
+}
+
+/* The caller must have pinned the task */
+static struct file *
+get_file_raw_ptr(struct task_struct *task, unsigned int idx)
+{
+        struct file *file = NULL;
+
+        task_lock(task);
+        rcu_read_lock();
+
+        if (task->files)
+                file = fcheck_files(task->files, idx);
+
+        rcu_read_unlock();
+        task_unlock(task);
+
+        return file;
+}
+
+static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
+{
+        if (likely(m2 != m1))
+                mutex_unlock(m2);
+        mutex_unlock(m1);
+}
+
+static int kcmp_lock(struct mutex *m1, struct mutex *m2)
+{
+        int err;
+
+        if (m2 > m1)
+                swap(m1, m2);
+
+        err = mutex_lock_killable(m1);
+        if (!err && likely(m1 != m2)) {
+                err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
+                if (err)
+                        mutex_unlock(m1);
+        }
+
+        return err;
+}
+
+SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
+                unsigned long, idx1, unsigned long, idx2)
+{
+        struct task_struct *task1, *task2;
+        int ret;
+
+        rcu_read_lock();
+
+        /*
+         * Tasks are looked up in caller's PID namespace only.
+         */
+        task1 = find_task_by_vpid(pid1);
+        task2 = find_task_by_vpid(pid2);
+        if (!task1 || !task2)
+                goto err_no_task;
+
+        get_task_struct(task1);
+        get_task_struct(task2);
+
+        rcu_read_unlock();
+
+        /*
+         * One should have enough rights to inspect task details.
+         */
+        ret = kcmp_lock(&task1->signal->cred_guard_mutex,
+                        &task2->signal->cred_guard_mutex);
+        if (ret)
+                goto err;
+        if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
+            !ptrace_may_access(task2, PTRACE_MODE_READ)) {
+                ret = -EPERM;
+                goto err_unlock;
+        }
+
+        switch (type) {
+        case KCMP_FILE: {
+                struct file *filp1, *filp2;
+
+                filp1 = get_file_raw_ptr(task1, idx1);
+                filp2 = get_file_raw_ptr(task2, idx2);
+
+                if (filp1 && filp2)
+                        ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
+                else
+                        ret = -EBADF;
+                break;
+        }
+        case KCMP_VM:
+                ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
+                break;
+        case KCMP_FILES:
+                ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
+                break;
+        case KCMP_FS:
+                ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
+                break;
+        case KCMP_SIGHAND:
+                ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
+                break;
+        case KCMP_IO:
+                ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
+                break;
+        case KCMP_SYSVSEM:
+#ifdef CONFIG_SYSVIPC
+                ret = kcmp_ptr(task1->sysvsem.undo_list,
+                               task2->sysvsem.undo_list,
+                               KCMP_SYSVSEM);
+#else
+                ret = -EOPNOTSUPP;
+#endif
+                break;
+        default:
+                ret = -EINVAL;
+                break;
+        }
+
+err_unlock:
+        kcmp_unlock(&task1->signal->cred_guard_mutex,
+                    &task2->signal->cred_guard_mutex);
+err:
+        put_task_struct(task1);
+        put_task_struct(task2);
+
+        return ret;
+
+err_no_task:
+        rcu_read_unlock();
+        return -ESRCH;
+}
+
+static __init int kcmp_cookies_init(void)
+{
+        int i;
+
+        get_random_bytes(cookies, sizeof(cookies));
+
+        for (i = 0; i < KCMP_TYPES; i++)
+                cookies[i][1] |= (~(~0UL >>  1) | 1);
+
+        return 0;
+}
+arch_initcall(kcmp_cookies_init);