fault-inject: simplify access check for fail-nth

The fail-nth file is created with 0666 and the access is permitted if
and only if the task is current.

This file is owned by the currnet user.  So we can create it with 0644
and allow the owner to write it.  This enables to watch the status of
task->fail_nth from another processes.

[akinobu.mita@gmail.com: don't convert unsigned type value as signed int]
  Link: http://lkml.kernel.org/r/1492444483-9239-1-git-send-email-akinobu.mita@gmail.com
[akinobu.mita@gmail.com: avoid unwanted data race to task->fail_nth]
  Link: http://lkml.kernel.org/r/1499962492-8931-1-git-send-email-akinobu.mita@gmail.com
Link: http://lkml.kernel.org/r/1491490561-10485-5-git-send-email-akinobu.mita@gmail.com
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -1363,16 +1363,16 @@ static ssize_t proc_fail_nth_write(struct file *file, const char __user *buf,
 	int err;
 	unsigned int n;
 
-	task = get_proc_task(file_inode(file));
-	if (!task)
-		return -ESRCH;
-	put_task_struct(task);
-	if (task != current)
-		return -EPERM;
 	err = kstrtouint_from_user(buf, count, 0, &n);
 	if (err)
 		return err;
-	current->fail_nth = n;
+
+	task = get_proc_task(file_inode(file));
+	if (!task)
+		return -ESRCH;
+	WRITE_ONCE(task->fail_nth, n);
+	put_task_struct(task);
+
 	return count;
 }
 
@@ -1386,11 +1386,10 @@ static ssize_t proc_fail_nth_read(struct file *file, char __user *buf,
 	task = get_proc_task(file_inode(file));
 	if (!task)
 		return -ESRCH;
-	put_task_struct(task);
-	if (task != current)
-		return -EPERM;
-	len = snprintf(numbuf, sizeof(numbuf), "%u\n", task->fail_nth);
+	len = snprintf(numbuf, sizeof(numbuf), "%u\n",
+			READ_ONCE(task->fail_nth));
 	len = simple_read_from_buffer(buf, count, ppos, numbuf, len);
+	put_task_struct(task);
 
 	return len;
 }
@@ -3355,11 +3354,7 @@ static const struct pid_entry tid_base_stuff[] = {
 #endif
 #ifdef CONFIG_FAULT_INJECTION
 	REG("make-it-fail", S_IRUGO|S_IWUSR, proc_fault_inject_operations),
-	/*
-	 * Operations on the file check that the task is current,
-	 * so we create it with 0666 to support testing under unprivileged user.
-	 */
-	REG("fail-nth", 0666, proc_fail_nth_operations),
+	REG("fail-nth", 0644, proc_fail_nth_operations),
 #endif
 #ifdef CONFIG_TASK_IO_ACCOUNTING
 	ONE("io",	S_IRUSR, proc_tid_io_accounting),

lib/fault-inject.c

@@ -107,9 +107,12 @@ static inline bool fail_stacktrace(struct fault_attr *attr)
 
 bool should_fail(struct fault_attr *attr, ssize_t size)
 {
-	if (in_task() && current->fail_nth) {
-		if (--current->fail_nth == 0)
+	if (in_task()) {
+		unsigned int fail_nth = READ_ONCE(current->fail_nth);
+
+		if (fail_nth && !WRITE_ONCE(current->fail_nth, fail_nth - 1))
 			goto fail;
+
 		return false;
 	}