[PATCH 4.19 80/93] x86/apic/x2apic: Fix a NULL pointer deref when handling a dying cpu

[Greg Kroah-Hartman]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

commit 7a22e03b0c02988e91003c505b34d752a51de344 upstream.

Check that the per-cpu cluster mask pointer has been set prior to
clearing a dying cpu's bit.  The per-cpu pointer is not set until the
target cpu reaches smp_callin() during CPUHP_BRINGUP_CPU, whereas the
teardown function, x2apic_dead_cpu(), is associated with the earlier
CPUHP_X2APIC_PREPARE.  If an error occurs before the cpu is awakened,
e.g. if do_boot_cpu() itself fails, x2apic_dead_cpu() will dereference
the NULL pointer and cause a panic.

  smpboot: do_boot_cpu failed(-22) to wakeup CPU#1
  BUG: kernel NULL pointer dereference, address: 0000000000000008
  RIP: 0010:x2apic_dead_cpu+0x1a/0x30
  Call Trace:
   cpuhp_invoke_callback+0x9a/0x580
   _cpu_up+0x10d/0x140
   do_cpu_up+0x69/0xb0
   smp_init+0x63/0xa9
   kernel_init_freeable+0xd7/0x229
   ? rest_init+0xa0/0xa0
   kernel_init+0xa/0x100
   ret_from_fork+0x35/0x40

Fixes: 023a611748fd5 ("x86/apic/x2apic: Simplify cluster management")
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20191001205019.5789-1-sean.j.christopherson@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/kernel/apic/x2apic_cluster.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/apic/x2apic_cluster.c
+++ b/arch/x86/kernel/apic/x2apic_cluster.c
@@ -158,7 +158,8 @@ static int x2apic_dead_cpu(unsigned int
 {
 	struct cluster_mask *cmsk = per_cpu(cluster_masks, dead_cpu);
 
-	cpumask_clear_cpu(dead_cpu, &cmsk->mask);
+	if (cmsk)
+		cpumask_clear_cpu(dead_cpu, &cmsk->mask);
 	free_cpumask_var(per_cpu(ipi_mask, dead_cpu));
 	return 0;
 }