kthread: Don't allocate kthread_struct for init and umh

If kthread_is_per_cpu runs concurrently with free_kthread_struct the
kthread_struct that was just freed may be read from.

This bug was introduced by commit 40966e316f86 ("kthread: Ensure
struct kthread is present for all kthreads").  When kthread_struct
started to be allocated for all tasks that have PF_KTHREAD set.  This
in turn required the kthread_struct to be freed in kernel_execve and
violated the assumption that kthread_struct will have the same
lifetime as the task.

Looking a bit deeper this only applies to callers of kernel_execve
which is just the init process and the user mode helper processes.
These processes really don't want to be kernel threads but are for
historical reasons.  Mostly that copy_thread does not know how to take
a kernel mode function to the process with for processes without
PF_KTHREAD or PF_IO_WORKER set.

Solve this by not allocating kthread_struct for the init process and
the user mode helper processes.

This is done by adding a kthread member to struct kernel_clone_args.
Setting kthread in fork_idle and kernel_thread.  Adding
user_mode_thread that works like kernel_thread except it does not set
kthread.  In fork only allocating the kthread_struct if .kthread is set.

I have looked at kernel/kthread.c and since commit 40966e316f86
("kthread: Ensure struct kthread is present for all kthreads") there
have been no assumptions added that to_kthread or __to_kthread will
not return NULL.

There are a few callers of to_kthread or __to_kthread that assume a
non-NULL struct kthread pointer will be returned.  These functions are
kthread_data(), kthread_parmme(), kthread_exit(), kthread(),
kthread_park(), kthread_unpark(), kthread_stop().  All of those functions
can reasonably expected to be called when it is know that a task is a
kthread so that assumption seems reasonable.

Cc: stable@vger.kernel.org
Fixes: 40966e316f86 ("kthread: Ensure struct kthread is present for all kthreads")
Reported-by: Максим Кутявин <maximkabox13@gmail.com>
Link: https://lkml.kernel.org/r/20220506141512.516114-1-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 719c9a6cac8d..4492266935dd 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -32,6 +32,7 @@ struct kernel_clone_args {
 	size_t set_tid_size;
 	int cgroup;
 	int io_thread;
+	int kthread;
 	struct cgroup *cgrp;
 	struct css_set *cset;
 };
@@ -89,6 +90,7 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node);
 struct task_struct *fork_idle(int);
 struct mm_struct *copy_init_mm(void);
 extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
+extern pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags);
 extern long kernel_wait4(pid_t, int __user *, int, struct rusage *);
 int kernel_wait(pid_t pid, int *stat);