Re: [PATCH 0/7] fork: Make init and umh ordinary tasks

[Qian Cai]

On Fri, May 06, 2022 at 09:11:36AM -0500, Eric W. Biederman wrote:
> 
> 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.
> 
> I believe my first patch in this series is enough to fix the bug
> and is simple enough and obvious 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.
> 
> There is one rough spot in this change.  In the init process before the
> user space init process is exec'd there is a lot going on.  I have found
> when async_schedule_domain is low on memory or has more than 32K callers
> executing do_populate_rootfs will now run in a user space thread making
> flush_delayed_fput meaningless, and __fput_sync is unusable.  I solved
> this as I did in usermode_driver.c with an added explicit task_work_run.
> I point this out as I have seen some talk about making flushing file
> handles more explicit.

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

 CPU: 72 PID: 1 Comm: swapper/0 Not tainted 5.18.0-rc6-next-20220509-dirty #29
 Call trace:
  dump_backtrace
  show_stack
  dump_stack_lvl
  print_report
  kasan_report
  kasan_check_range
  __kasan_check_read
  task_nr_scan_windows.isra.0
  task_scan_start
  task_scan_min at /home/user/linux/kernel/sched/fair.c:1144
  (inlined by) task_scan_start at /home/user/linux/kernel/sched/fair.c:1150
  task_tick_fair
  task_tick_numa at /home/user/linux/kernel/sched/fair.c:2944
  (inlined by) task_tick_fair at /home/user/linux/kernel/sched/fair.c:11186
  scheduler_tick
  update_process_times
  tick_periodic
  tick_handle_periodic
  arch_timer_handler_phys
  handle_percpu_devid_irq
  generic_handle_domain_irq
  gic_handle_irq
  call_on_irq_stack
  do_interrupt_handler
  el1_interrupt
  el1h_64_irq_handler
  el1h_64_irq
  split_page
  make_alloc_exact
  alloc_pages_exact_nid
  init_section_page_ext
  page_ext_init
  kernel_init_freeable
  kernel_init
  ret_from_fork
 ==================================================================
 Disabling lock debugging due to kernel taint
 Unable to handle kernel paging request at virtual address dfff80000000007a
 KASAN: null-ptr-deref in range [0x00000000000003d0-0x00000000000003d7]
 Mem abort info:
   ESR = 0x0000000096000004
   EC = 0x25: DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
   FSC = 0x04: level 0 translation fault
 Data abort info:
   ISV = 0, ISS = 0x00000004
   CM = 0, WnR = 0
 [dfff80000000007a] address between user and kernel address ranges
 Internal error: Oops: 96000004 [#1] PREEMPT SMP
 Modules linked in:
 CPU: 72 PID: 1 Comm: swapper/0 Tainted: G    B             5.18.0-rc6-next-20220509-dirty #29
 pstate: 404000c9 (nZcv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : task_nr_scan_windows.isra.0
 lr : task_nr_scan_windows.isra.0
 sp : ffff800008487cb0
 x29: ffff800008487cb0 x28: ffff07ff89728040 x27: 000000003bc47ee0
 x26: ffff08367f088980 x25: 1fffe0fff12e525f x24: ffff07ff897292f8
 x23: ffff07ff89728040 x22: 1fffe0fff12e5262 x21: 0000000000010000
 x20: 00000000000003d0 x19: 0000000000000000 x18: ffffdd41783f7d1c
 x17: 3d3d3d3d3d3d3d3d x16: 3d3d3d3d3d3d3d3d x15: 3d3d3d3d3d3d3d3d
 x14: 3d3d3d3d3d3d3d3d x13: 746e696174206c65 x12: ffff7ba82f3b98b5
 x11: 1ffffba82f3b98b4 x10: ffff7ba82f3b98b4 x9 : dfff800000000000
 x8 : ffffdd4179dcc5a7 x7 : 0000000000000001 x6 : ffff7ba82f3b98b4
 x5 : ffffdd4179dcc5a0 x4 : ffff7ba82f3b98b5 x3 : ffffdd4171de2b14
 x2 : 0000000000000001 x1 : 000000000000007a x0 : dfff800000000000
 Call trace:
  task_nr_scan_windows.isra.0
  task_scan_start
  task_tick_fair
  scheduler_tick
  update_process_times
  tick_periodic
  tick_handle_periodic
  arch_timer_handler_phys
  handle_percpu_devid_irq
  generic_handle_domain_irq
  gic_handle_irq
  call_on_irq_stack
  do_interrupt_handler
  el1_interrupt
  el1h_64_irq_handler
  el1h_64_irq
  split_page
  make_alloc_exact
  alloc_pages_exact_nid
  init_section_page_ext
  page_ext_init
  kernel_init_freeable
  kernel_init
  ret_from_fork
 Code: d343fe81 d2d00000 f2fbffe0 53185eb5 (38e06820)