During hotplug, the stolen time data structure is unmapped and memset.
There is a possibility of the timer IRQ being triggered before memset
and stolen time is getting updated as part of this timer IRQ handler. This
causes the below crash in timer handler -

  [ 3457.473139][    C5] Unable to handle kernel paging request at virtual address ffffffc03df05148
  ...
  [ 3458.154398][    C5] Call trace:
  [ 3458.157648][    C5]  para_steal_clock+0x30/0x50
  [ 3458.162319][    C5]  irqtime_account_process_tick+0x30/0x194
  [ 3458.168148][    C5]  account_process_tick+0x3c/0x280
  [ 3458.173274][    C5]  update_process_times+0x5c/0xf4
  [ 3458.178311][    C5]  tick_sched_timer+0x180/0x384
  [ 3458.183164][    C5]  __run_hrtimer+0x160/0x57c
  [ 3458.187744][    C5]  hrtimer_interrupt+0x258/0x684
  [ 3458.192698][    C5]  arch_timer_handler_virt+0x5c/0xa0
  [ 3458.198002][    C5]  handle_percpu_devid_irq+0xdc/0x414
  [ 3458.203385][    C5]  handle_domain_irq+0xa8/0x168
  [ 3458.208241][    C5]  gic_handle_irq.34493+0x54/0x244
  [ 3458.213359][    C5]  call_on_irq_stack+0x40/0x70
  [ 3458.218125][    C5]  do_interrupt_handler+0x60/0x9c
  [ 3458.223156][    C5]  el1_interrupt+0x34/0x64
  [ 3458.227560][    C5]  el1h_64_irq_handler+0x1c/0x2c
  [ 3458.232503][    C5]  el1h_64_irq+0x7c/0x80
  [ 3458.236736][    C5]  free_vmap_area_noflush+0x108/0x39c
  [ 3458.242126][    C5]  remove_vm_area+0xbc/0x118
  [ 3458.246714][    C5]  vm_remove_mappings+0x48/0x2a4
  [ 3458.251656][    C5]  __vunmap+0x154/0x278
  [ 3458.255796][    C5]  stolen_time_cpu_down_prepare+0xc0/0xd8
  [ 3458.261542][    C5]  cpuhp_invoke_callback+0x248/0xc34
  [ 3458.266842][    C5]  cpuhp_thread_fun+0x1c4/0x248
  [ 3458.271696][    C5]  smpboot_thread_fn+0x1b0/0x400
  [ 3458.276638][    C5]  kthread+0x17c/0x1e0
  [ 3458.280691][    C5]  ret_from_fork+0x10/0x20

As a fix, introduce rcu lock to update stolen time structure.

Fixes: 75df529b ("arm64: paravirt: Initialize steal time when cpu is online")
Cc: stable@vger.kernel.org
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Prakruthi Deepak Heragu <quic_pheragu@quicinc.com>
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Reviewed-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Link: https://lore.kernel.org/r/20220513174654.362169-1-quic_eberman@quicinc.comSigned-off-by: Will Deacon <will@kernel.org>