Re: [PATCH v2] arm64: paravirt: Use RCU read locks to guard stolen_time

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On 04.05.22 11:45, Will Deacon wrote:
On Thu, Apr 28, 2022 at 11:35:36AM -0700, Elliot Berman wrote:
From: Prakruthi Deepak Heragu <quic_pheragu@xxxxxxxxxxx>

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: 75df529bec91 ("arm64: paravirt: Initialize steal time when cpu is online")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Prakruthi Deepak Heragu <quic_pheragu@xxxxxxxxxxx>
Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
---
Changes since v1: https://lore.kernel.org/all/20220420204417.155194-1-quic_eberman@xxxxxxxxxxx/
  - Use RCU instead of disabling interrupts

  arch/arm64/kernel/paravirt.c | 24 +++++++++++++++++++-----
  1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
index 75fed4460407..e724ea3d86f0 100644
--- a/arch/arm64/kernel/paravirt.c
+++ b/arch/arm64/kernel/paravirt.c
@@ -52,7 +52,9 @@ early_param("no-steal-acc", parse_no_stealacc);
  /* return stolen time in ns by asking the hypervisor */
  static u64 para_steal_clock(int cpu)
  {
+	struct pvclock_vcpu_stolen_time *kaddr = NULL;
  	struct pv_time_stolen_time_region *reg;
+	u64 ret = 0;
reg = per_cpu_ptr(&stolen_time_region, cpu); @@ -61,28 +63,38 @@ static u64 para_steal_clock(int cpu)
  	 * online notification callback runs. Until the callback
  	 * has run we just return zero.
  	 */
-	if (!reg->kaddr)
+	rcu_read_lock();
+	kaddr = rcu_dereference(reg->kaddr);
+	if (!kaddr) {
+		rcu_read_unlock();
  		return 0;
+	}
- return le64_to_cpu(READ_ONCE(reg->kaddr->stolen_time));
+	ret = le64_to_cpu(READ_ONCE(kaddr->stolen_time));

Is this READ_ONCE() still required now?

Yes, as it might be called for another cpu than the current one.
stolen_time might just be updated, so you want to avoid load tearing.


Juergen

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