proc: revalidate kernel thread inodes to root:root

task_dump_owner() has the following code:

	mm = task->mm;
	if (mm) {
		if (get_dumpable(mm) != SUID_DUMP_USER) {
			uid = ...
		}
	}

Check for ->mm is buggy -- kernel thread might be borrowing mm
and inode will go to some random uid:gid pair.

Link: http://lkml.kernel.org/r/20180412220109.GA20978@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -1693,6 +1693,12 @@ void task_dump_owner(struct task_struct *task, umode_t mode,
 	kuid_t uid;
 	kgid_t gid;
 
+	if (unlikely(task->flags & PF_KTHREAD)) {
+		*ruid = GLOBAL_ROOT_UID;
+		*rgid = GLOBAL_ROOT_GID;
+		return;
+	}
+
 	/* Default to the tasks effective ownership */
 	rcu_read_lock();
 	cred = __task_cred(task);