From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: syscalls, x86: Fix __NR_kcmp execve race and potential NULL dereference

Plain ptrace_may_access() check used in kcmp is not safe
against race with execve(setuid_app), so we need to grab
cred_guard_mutex and keep it until kcmp is finished.

Also task->files may be nil, better to use task_lock
and fcheck_files helpers instead of direct file_lock
usage.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
diff -u linux-2.6.git/kernel/kcmp.c linux-2.6.git/kernel/kcmp.c
--- linux-2.6.git/kernel/kcmp.c
+++ linux-2.6.git/kernel/kcmp.c
@@ -44,20 +44,38 @@
 static struct file *
 get_file_raw_ptr(struct task_struct *task, unsigned int idx)
 {
-	struct fdtable *fdt;
-	struct file *file;
+	struct file *file = NULL;
 
-	spin_lock(&task->files->file_lock);
-	fdt = files_fdtable(task->files);
-	if (idx < fdt->max_fds)
-		file = fdt->fd[idx];
-	else
-		file = NULL;
-	spin_unlock(&task->files->file_lock);
+	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 access_unlock(struct task_struct *task)
+{
+	mutex_unlock(&task->signal->cred_guard_mutex);
+}
+
+static int access_trylock(struct task_struct *task)
+{
+	if (!mutex_trylock(&task->signal->cred_guard_mutex))
+		return -EBUSY;
+
+	if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
+		mutex_unlock(&task->signal->cred_guard_mutex);
+		return -EPERM;
+	}
+
+	return 0;
+}
+
 SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
 		unsigned long, idx1, unsigned long, idx2)
 {
@@ -82,11 +100,12 @@
 	/*
 	 * One should have enough rights to inspect task details.
 	 */
-	if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
-	    !ptrace_may_access(task2, PTRACE_MODE_READ)) {
-		ret = -EACCES;
+	ret = access_trylock(task1);
+	if (ret)
 		goto err;
-	}
+	ret = access_trylock(task2);
+	if (ret)
+		goto err_unlock;
 
 	switch (type) {
 	case KCMP_FILE: {
@@ -130,6 +149,9 @@
 		break;
 	}
 
+	access_unlock(task2);
+err_unlock:
+	access_unlock(task1);
 err:
 	put_task_struct(task1);
 	put_task_struct(task2);