arm64: paravirt: Use RCU read locks to guard stolen_time

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: 75df529bec ("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.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/paravirt.c

@@ -35,7 +35,7 @@ static u64 native_steal_clock(int cpu)
 DEFINE_STATIC_CALL(pv_steal_clock, native_steal_clock);
 
 struct pv_time_stolen_time_region {
-	struct pvclock_vcpu_stolen_time *kaddr;
+	struct pvclock_vcpu_stolen_time __rcu *kaddr;
 };
 
 static DEFINE_PER_CPU(struct pv_time_stolen_time_region, stolen_time_region);
@@ -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 +63,37 @@ 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));
+	rcu_read_unlock();
+	return ret;
 }
 
 static int stolen_time_cpu_down_prepare(unsigned int cpu)
 {
+	struct pvclock_vcpu_stolen_time *kaddr = NULL;
 	struct pv_time_stolen_time_region *reg;
 
 	reg = this_cpu_ptr(&stolen_time_region);
 	if (!reg->kaddr)
 		return 0;
 
-	memunmap(reg->kaddr);
-	memset(reg, 0, sizeof(*reg));
+	kaddr = rcu_replace_pointer(reg->kaddr, NULL, true);
+	synchronize_rcu();
+	memunmap(kaddr);
 
 	return 0;
 }
 
 static int stolen_time_cpu_online(unsigned int cpu)
 {
+	struct pvclock_vcpu_stolen_time *kaddr = NULL;
 	struct pv_time_stolen_time_region *reg;
 	struct arm_smccc_res res;
 
@@ -93,17 +104,19 @@ static int stolen_time_cpu_online(unsigned int cpu)
 	if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
 		return -EINVAL;
 
-	reg->kaddr = memremap(res.a0,
+	kaddr = memremap(res.a0,
 			      sizeof(struct pvclock_vcpu_stolen_time),
 			      MEMREMAP_WB);
 
+	rcu_assign_pointer(reg->kaddr, kaddr);
+
 	if (!reg->kaddr) {
 		pr_warn("Failed to map stolen time data structure\n");
 		return -ENOMEM;
 	}
 
-	if (le32_to_cpu(reg->kaddr->revision) != 0 ||
-	    le32_to_cpu(reg->kaddr->attributes) != 0) {
+	if (le32_to_cpu(kaddr->revision) != 0 ||
+	    le32_to_cpu(kaddr->attributes) != 0) {
 		pr_warn_once("Unexpected revision or attributes in stolen time data\n");
 		return -ENXIO;
 	}