This set of changes updates init and user mode helper tasks to be
ordinary user mode tasks.

In commit 40966e316f86 ("kthread: Ensure struct kthread is present for
all kthreads") caused init and the user mode helper threads that call
kernel_execve to have struct kthread allocated for them.  This struct
kthread going away during execve in turned made a use after free of
struct kthread possible.

The commit 343f4c49f243 ("kthread: Don't allocate kthread_struct for
init and umh") is enough to fix the use after free and is simple enough
to be backportable.

The rest of the changes pass struct kernel_clone_args to clean things
up and cause the code to make sense.

In making init and the user mode helpers tasks purely user mode tasks
I ran into two complications.  The function task_tick_numa was
detecting tasks without an mm by testing for the presence of
PF_KTHREAD.  The initramfs code in populate_initrd_image was using
flush_delayed_fput to ensuere the closing of all it's file descriptors
was complete, and flush_delayed_fput does not work in a userspace thread.

I have looked and looked and more complications and in my code review
I have not found any, and neither has anyone else with the code sitting
in linux-next.

Link: https://lkml.kernel.org/r/87mtfu4up3.fsf@email.froward.int.ebiederm.org

Eric W. Biederman (8):
      kthread: Don't allocate kthread_struct for init and umh
      fork: Pass struct kernel_clone_args into copy_thread
      fork: Explicity test for idle tasks in copy_thread
      fork: Generalize PF_IO_WORKER handling
      init: Deal with the init process being a user mode process
      fork: Explicitly set PF_KTHREAD
      fork: Stop allowing kthreads to call execve
      sched: Update task_tick_numa to ignore tasks without an mm

 arch/alpha/kernel/process.c      | 13 ++++++------
 arch/arc/kernel/process.c        | 13 ++++++------
 arch/arm/kernel/process.c        | 12 ++++++-----
 arch/arm64/kernel/process.c      | 12 ++++++-----
 arch/csky/kernel/process.c       | 15 ++++++-------
 arch/h8300/kernel/process.c      | 10 ++++-----
 arch/hexagon/kernel/process.c    | 12 ++++++-----
 arch/ia64/kernel/process.c       | 15 +++++++------
 arch/m68k/kernel/process.c       | 12 ++++++-----
 arch/microblaze/kernel/process.c | 12 ++++++-----
 arch/mips/kernel/process.c       | 13 ++++++------
 arch/nios2/kernel/process.c      | 12 ++++++-----
 arch/openrisc/kernel/process.c   | 12 ++++++-----
 arch/parisc/kernel/process.c     | 18 +++++++++-------
 arch/powerpc/kernel/process.c    | 15 +++++++------
 arch/riscv/kernel/process.c      | 12 ++++++-----
 arch/s390/kernel/process.c       | 12 ++++++-----
 arch/sh/kernel/process_32.c      | 12 ++++++-----
 arch/sparc/kernel/process_32.c   | 12 ++++++-----
 arch/sparc/kernel/process_64.c   | 12 ++++++-----
 arch/um/kernel/process.c         | 15 +++++++------
 arch/x86/include/asm/fpu/sched.h |  2 +-
 arch/x86/include/asm/switch_to.h |  8 +++----
 arch/x86/kernel/fpu/core.c       |  4 ++--
 arch/x86/kernel/process.c        | 18 +++++++++-------
 arch/xtensa/kernel/process.c     | 17 ++++++++-------
 fs/exec.c                        |  8 ++++---
 include/linux/sched/task.h       |  8 +++++--
 init/initramfs.c                 |  2 ++
 init/main.c                      |  2 +-
 kernel/fork.c                    | 46 +++++++++++++++++++++++++++++++++-------
 kernel/sched/fair.c              |  2 +-
 kernel/umh.c                     |  6 +++---
 33 files changed, 234 insertions(+), 160 deletions(-)

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

sched: Update task_tick_numa to ignore tasks without an mm

Qian Cai <quic_qiancai@quicinc.com> wrote:
> Reverting the last 3 commits of the series fixed a boot crash.
>
> 1b2552cbdbe0 fork: Stop allowing kthreads to call execve
> 753550eb0ce1 fork: Explicitly set PF_KTHREAD
> 68d85f0a33b0 init: Deal with the init process being a user mode process
>
>  BUG: KASAN: null-ptr-deref in task_nr_scan_windows.isra.0
>  arch_atomic_long_read at ./include/linux/atomic/atomic-long.h:29
>  (inlined by) atomic_long_read at ./include/linux/atomic/atomic-instrumented.h:1266
>  (inlined by) get_mm_counter at ./include/linux/mm.h:1996
>  (inlined by) get_mm_rss at ./include/linux/mm.h:2049
>  (inlined by) task_nr_scan_windows at kernel/sched/fair.c:1123
>  Read of size 8 at addr 00000000000003d0 by task swapper/0/1

With the change to init and the user mode helper processes to not have
PF_KTHREAD set before they call kernel_execve the PF_KTHREAD test in
task_tick_numa became insufficient to detect all tasks that have
"->mm == NULL".  Correct that by testing for "->mm == NULL" directly.

Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Tested-by: Qian Cai <quic_qiancai@quicinc.com>
Fixes: 1b2552cbdbe0 ("fork: Stop allowing kthreads to call execve")
Link: https://lkml.kernel.org/r/87r150ug1l.fsf_-_@email.froward.int.ebiederm.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>