userfaultd: Possible deadlock

Hi Christoph,

I think that a recent commit from you introduced the syzbot complaint shown below. Can you have a look at this?

I'm referring to commit ae62c16e105a ("userfaultfd: disable irqs when taking the waitqueue lock". That commit went upstream in kernel v4.20.

Thanks,

Bart.

syzbot has found the following crash on:

HEAD commit:    5eeb63359b1e Merge tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
console output: https://syzkaller-buganizer.googleplex.com/text?tag=CrashLog&id=abfb8cf5ee75596c3cec97fbca90007f7c572fd5
kernel config:  https://syzkaller-buganizer.googleplex.com/text?tag=Config&id=2f236acd4d07d3e9680112eeef52906988664d3b
dashboard link: https://syzkaller.appspot.com/bug?extid=554a124791d98722a022
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

See http://go/syzbot for details on how to handle this bug.

=====================================================WARNING: SOFTIRQ-safe ->SOFTIRQ-unsafe lock order detected
5.0.0-rc4+- #56 Not tainted
-----------------------------------------------------
syz-executor5/9727 [HC0[0]SC0[0]HE0:SE1]is trying to acquire:
00000000a4278d31 (&ctx->fault_pending_wqh){++-.} at: spin_lock include/linux/spinlock.h:329 [inline]00000000a4278d31 (&ctx->fault_pending_wqh){++-.} at: userfaultfd_ctx_read fs/userfaultfd.c:1040 [inline]00000000a4278d31 (&ctx->fault_pending_wqh){++-.} at: userfaultfd_read+-0x540/0x1940 fs/userfaultfd.c:1198

and this task is already holding:
000000000e5b4350 (&ctx->fd_wqh){...} at: spin_lock_irq include/linux/spinlock.h:354 [inline]000000000e5b4350 (&ctx->fd_wqh){...} at: userfaultfd_ctx_read fs/userfaultfd.c:1036 [inline]000000000e5b4350 (&ctx->fd_wqh){...} at: userfaultfd_read+-0x27a/0x1940 fs/userfaultfd.c:1198 which would create a new lock dependency:
(&ctx->fd_wqh){...}->(&ctx->fault_pending_wqh){++-.}
but this new dependency connects a SOFTIRQ-irq-safe lock:
(&&ctx->ctx_lock)->rlock){.-.}
... which became SOFTIRQ-irq-safe at:
lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841

  __raw_spin_lock_irq include/linux/spinlock_api_smp.h:128 [inline]  _raw_spin_lock_irq+-0x60/0x80 kernel/locking/spinlock.c:160
  spin_lock_irq include/linux/spinlock.h:354 [inline]  free_ioctx_users+-0x2d/0x4a0 fs/aio.c:610
  percpu_ref_put_many include/linux/percpu-refcount.h:285 [inline]  percpu_ref_put include/linux/percpu-refcount.h:301 [inline]  percpu_ref_call_confirm_rcu lib/percpu-refcount.c:123 [inline]  percpu_ref_switch_to_atomic_rcu+-0x3e7/0x520 lib/percpu-refcount.c:158
  __rcu_reclaim kernel/rcu/rcu.h:240 [inline]  rcu_do_batch kernel/rcu/tree.c:2452 [inline]  invoke_rcu_callbacks kernel/rcu/tree.c:2773 [inline]  rcu_process_callbacks+-0x928/0x1390 kernel/rcu/tree.c:2754
  __do_softirq+-0x266/0x95a kernel/softirq.c:292
  run_ksoftirqd kernel/softirq.c:654 [inline]  run_ksoftirqd+-0x8e/0x110 kernel/softirq.c:646
  smpboot_thread_fn+-0x6ab/0xa10 kernel/smpboot.c:164
  kthread+-0x357/0x430 kernel/kthread.c:246
  ret_from_fork+-0x3a/0x50 arch/x86/entry/entry_64.S:352

to a SOFTIRQ-irq-unsafe lock:
(&ctx->fault_pending_wqh){++-.}
... which became SOFTIRQ-irq-unsafe at:
...
lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841

  __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]  _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
  spin_lock include/linux/spinlock.h:329 [inline]  userfaultfd_release+-0x497/0x6d0 fs/userfaultfd.c:916
  __fput+-0x2df/0x8d0 fs/file_table.c:278
  ____fput+-0x16/0x20 fs/file_table.c:309
  task_work_run+-0x14a/0x1c0 kernel/task_work.c:113
  tracehook_notify_resume include/linux/tracehook.h:188 [inline]  exit_to_usermode_loop+-0x273/0x2c0 arch/x86/entry/common.c:166
  prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]  syscall_return_slowpath arch/x86/entry/common.c:268 [inline]  do_syscall_64+-0x52d/0x610 arch/x86/entry/common.c:293
  entry_SYSCALL_64_after_hwframe+-0x49/0xbe

other info that might help us debug this:

Chain exists of:
  &&ctx->ctx_lock)->rlock -->&ctx->fd_wqh -->&ctx->fault_pending_wqh

Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----

lock(&ctx->fault_pending_wqh); local_irq_disable(); lock(&&ctx->ctx_lock)->rlock); lock(&ctx->fd_wqh); <Interrupt> lock(&&ctx->ctx_lock)->rlock);
***DEADLOCK ***
1 lock held by syz-executor5/9727:
#0: 000000000e5b4350 (&ctx->fd_wqh){...} at: spin_lock_irq include/linux/spinlock.h:354 [inline] #0: 000000000e5b4350 (&ctx->fd_wqh){...} at: userfaultfd_ctx_read fs/userfaultfd.c:1036 [inline] #0: 000000000e5b4350 (&ctx->fd_wqh){...} at: userfaultfd_read+-0x27a/0x1940 fs/userfaultfd.c:1198

the dependencies between SOFTIRQ-irq-safe lock and the holding lock:
->(&&ctx->ctx_lock)->rlock){.-.}{ IN-SOFTIRQ-W at:

                      lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
                      __raw_spin_lock_irq include/linux/spinlock_api_smp.h:128 [inline]                      _raw_spin_lock_irq+-0x60/0x80 kernel/locking/spinlock.c:160
                      spin_lock_irq include/linux/spinlock.h:354 [inline]                      free_ioctx_users+-0x2d/0x4a0 fs/aio.c:610
                      percpu_ref_put_many include/linux/percpu-refcount.h:285 [inline]                      percpu_ref_put include/linux/percpu-refcount.h:301 [inline]                      percpu_ref_call_confirm_rcu lib/percpu-refcount.c:123 [inline]                      percpu_ref_switch_to_atomic_rcu+-0x3e7/0x520 lib/percpu-refcount.c:158
                      __rcu_reclaim kernel/rcu/rcu.h:240 [inline]                      rcu_do_batch kernel/rcu/tree.c:2452 [inline]                      invoke_rcu_callbacks kernel/rcu/tree.c:2773 [inline]                      rcu_process_callbacks+-0x928/0x1390 kernel/rcu/tree.c:2754
                      __do_softirq+-0x266/0x95a kernel/softirq.c:292
                      run_ksoftirqd kernel/softirq.c:654 [inline]                      run_ksoftirqd+-0x8e/0x110 kernel/softirq.c:646
                      smpboot_thread_fn+-0x6ab/0xa10 kernel/smpboot.c:164
                      kthread+-0x357/0x430 kernel/kthread.c:246
                      ret_from_fork+-0x3a/0x50 arch/x86/entry/entry_64.S:352
    INITIAL USE at:
                     lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
                     __raw_spin_lock_irq include/linux/spinlock_api_smp.h:128 [inline]                     _raw_spin_lock_irq+-0x60/0x80 kernel/locking/spinlock.c:160
                     spin_lock_irq include/linux/spinlock.h:354 [inline]                     free_ioctx_users+-0x2d/0x4a0 fs/aio.c:610
                     percpu_ref_put_many include/linux/percpu-refcount.h:285 [inline]                     percpu_ref_put include/linux/percpu-refcount.h:301 [inline]                     percpu_ref_call_confirm_rcu lib/percpu-refcount.c:123 [inline]                     percpu_ref_switch_to_atomic_rcu+-0x3e7/0x520 lib/percpu-refcount.c:158
                     __rcu_reclaim kernel/rcu/rcu.h:240 [inline]                     rcu_do_batch kernel/rcu/tree.c:2452 [inline]                     invoke_rcu_callbacks kernel/rcu/tree.c:2773 [inline]                     rcu_process_callbacks+-0x928/0x1390 kernel/rcu/tree.c:2754
                     __do_softirq+-0x266/0x95a kernel/softirq.c:292
                     run_ksoftirqd kernel/softirq.c:654 [inline]                     run_ksoftirqd+-0x8e/0x110 kernel/softirq.c:646
                     smpboot_thread_fn+-0x6ab/0xa10 kernel/smpboot.c:164
                     kthread+-0x357/0x430 kernel/kthread.c:246
                     ret_from_fork+-0x3a/0x50 arch/x86/entry/entry_64.S:352
  }  ... key      at: [<ffffffff8a5760a0>]__key.51972+-0x0/0x40
  ... acquired at:
   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]   _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
   spin_lock include/linux/spinlock.h:329 [inline]   aio_poll fs/aio.c:1772 [inline]   __io_submit_one fs/aio.c:1875 [inline]   io_submit_one+-0xedf/0x1cf0 fs/aio.c:1908
   __do_sys_io_submit fs/aio.c:1953 [inline]   __se_sys_io_submit fs/aio.c:1923 [inline]   __x64_sys_io_submit+-0x1bd/0x580 fs/aio.c:1923
   do_syscall_64+-0x103/0x610 arch/x86/entry/common.c:290
   entry_SYSCALL_64_after_hwframe+-0x49/0xbe

->(&ctx->fd_wqh){...}{ INITIAL USE at:

                   lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
                   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]                   _raw_spin_lock_irqsave+-0x95/0xcd kernel/locking/spinlock.c:152
                   __wake_up_common_lock+-0xc7/0x190 kernel/sched/wait.c:120
                   __wake_up+-0xe/0x10 kernel/sched/wait.c:145
                   userfaultfd_release+-0x4f5/0x6d0 fs/userfaultfd.c:924
                   __fput+-0x2df/0x8d0 fs/file_table.c:278
                   ____fput+-0x16/0x20 fs/file_table.c:309
                   task_work_run+-0x14a/0x1c0 kernel/task_work.c:113
                   tracehook_notify_resume include/linux/tracehook.h:188 [inline]                   exit_to_usermode_loop+-0x273/0x2c0 arch/x86/entry/common.c:166
                   prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]                   syscall_return_slowpath arch/x86/entry/common.c:268 [inline]                   do_syscall_64+-0x52d/0x610 arch/x86/entry/common.c:293
                   entry_SYSCALL_64_after_hwframe+-0x49/0xbe
 } ... key      at: [<ffffffff8a575e20>]__key.44854+-0x0/0x40
 ... acquired at:
   lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]   _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
   spin_lock include/linux/spinlock.h:329 [inline]   userfaultfd_ctx_read fs/userfaultfd.c:1040 [inline]   userfaultfd_read+-0x540/0x1940 fs/userfaultfd.c:1198
   __vfs_read+-0x116/0x8c0 fs/read_write.c:416
   vfs_read+-0x194/0x3e0 fs/read_write.c:452
   ksys_read+-0xea/0x1f0 fs/read_write.c:578
   __do_sys_read fs/read_write.c:588 [inline]   __se_sys_read fs/read_write.c:586 [inline]   __x64_sys_read+-0x73/0xb0 fs/read_write.c:586
   do_syscall_64+-0x103/0x610 arch/x86/entry/common.c:290
   entry_SYSCALL_64_after_hwframe+-0x49/0xbe

the dependencies between the lock to be acquired
and SOFTIRQ-irq-unsafe lock:
->(&ctx->fault_pending_wqh){++-.}{ HARDIRQ-ON-W at:

                    lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
                    __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]                    _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
                    spin_lock include/linux/spinlock.h:329 [inline]                    userfaultfd_release+-0x497/0x6d0 fs/userfaultfd.c:916
                    __fput+-0x2df/0x8d0 fs/file_table.c:278
                    ____fput+-0x16/0x20 fs/file_table.c:309
                    task_work_run+-0x14a/0x1c0 kernel/task_work.c:113
                    tracehook_notify_resume include/linux/tracehook.h:188 [inline]                    exit_to_usermode_loop+-0x273/0x2c0 arch/x86/entry/common.c:166
                    prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]                    syscall_return_slowpath arch/x86/entry/common.c:268 [inline]                    do_syscall_64+-0x52d/0x610 arch/x86/entry/common.c:293
                    entry_SYSCALL_64_after_hwframe+-0x49/0xbe
   SOFTIRQ-ON-W at:
                    lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
                    __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]                    _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
                    spin_lock include/linux/spinlock.h:329 [inline]                    userfaultfd_release+-0x497/0x6d0 fs/userfaultfd.c:916
                    __fput+-0x2df/0x8d0 fs/file_table.c:278
                    ____fput+-0x16/0x20 fs/file_table.c:309
                    task_work_run+-0x14a/0x1c0 kernel/task_work.c:113
                    tracehook_notify_resume include/linux/tracehook.h:188 [inline]                    exit_to_usermode_loop+-0x273/0x2c0 arch/x86/entry/common.c:166
                    prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]                    syscall_return_slowpath arch/x86/entry/common.c:268 [inline]                    do_syscall_64+-0x52d/0x610 arch/x86/entry/common.c:293
                    entry_SYSCALL_64_after_hwframe+-0x49/0xbe
   INITIAL USE at:
                   lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
                   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]                   _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
                   spin_lock include/linux/spinlock.h:329 [inline]                   userfaultfd_release+-0x497/0x6d0 fs/userfaultfd.c:916
                   __fput+-0x2df/0x8d0 fs/file_table.c:278
                   ____fput+-0x16/0x20 fs/file_table.c:309
                   task_work_run+-0x14a/0x1c0 kernel/task_work.c:113
                   tracehook_notify_resume include/linux/tracehook.h:188 [inline]                   exit_to_usermode_loop+-0x273/0x2c0 arch/x86/entry/common.c:166
                   prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]                   syscall_return_slowpath arch/x86/entry/common.c:268 [inline]                   do_syscall_64+-0x52d/0x610 arch/x86/entry/common.c:293
                   entry_SYSCALL_64_after_hwframe+-0x49/0xbe
 } ... key      at: [<ffffffff8a575ee0>]__key.44851+-0x0/0x40
 ... acquired at:
   lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]   _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
   spin_lock include/linux/spinlock.h:329 [inline]   userfaultfd_ctx_read fs/userfaultfd.c:1040 [inline]   userfaultfd_read+-0x540/0x1940 fs/userfaultfd.c:1198
   __vfs_read+-0x116/0x8c0 fs/read_write.c:416
   vfs_read+-0x194/0x3e0 fs/read_write.c:452
   ksys_read+-0xea/0x1f0 fs/read_write.c:578
   __do_sys_read fs/read_write.c:588 [inline]   __se_sys_read fs/read_write.c:586 [inline]   __x64_sys_read+-0x73/0xb0 fs/read_write.c:586
   do_syscall_64+-0x103/0x610 arch/x86/entry/common.c:290
   entry_SYSCALL_64_after_hwframe+-0x49/0xbe


stack backtrace:
CPU: 1 PID: 9727 Comm: syz-executor5 Not tainted 5.0.0-rc4+- #56
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline] dump_stack+-0x172/0x1f0 lib/dump_stack.c:113
 print_bad_irq_dependency kernel/locking/lockdep.c:1573 [inline] check_usage.cold+-0x60f/0x940 kernel/locking/lockdep.c:1605
 check_irq_usage kernel/locking/lockdep.c:1661 [inline] check_prev_add_irq kernel/locking/lockdep_states.h:8 [inline] check_prev_add kernel/locking/lockdep.c:1871 [inline] check_prevs_add kernel/locking/lockdep.c:1979 [inline] validate_chain kernel/locking/lockdep.c:2350 [inline] __lock_acquire+-0x1f47/0x4700 kernel/locking/lockdep.c:3338
 lock_acquire+-0x16f/0x3f0 kernel/locking/lockdep.c:3841
 __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline] _raw_spin_lock+-0x2f/0x40 kernel/locking/spinlock.c:144
 spin_lock include/linux/spinlock.h:329 [inline] userfaultfd_ctx_read fs/userfaultfd.c:1040 [inline] userfaultfd_read+-0x540/0x1940 fs/userfaultfd.c:1198
 __vfs_read+-0x116/0x8c0 fs/read_write.c:416
 vfs_read+-0x194/0x3e0 fs/read_write.c:452
 ksys_read+-0xea/0x1f0 fs/read_write.c:578
 __do_sys_read fs/read_write.c:588 [inline] __se_sys_read fs/read_write.c:586 [inline] __x64_sys_read+-0x73/0xb0 fs/read_write.c:586
 do_syscall_64+-0x103/0x610 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+-0x49/0xbe