[PATCH] genirq: Fix NULL pointer dereference in resend_irqs()

[Yunfeng Ye <yeyunfeng@huawei.com>]

Oops occur when running qemu on arm64:

  Unable to handle kernel NULL pointer dereference at 0000000000000158
  Internal error: Oops: 96000004 [#1] SMP
  pc : resend_irqs+0x68/0xb0
  lr : resend_irqs+0x64/0xb0
  ...
  Call trace:
   resend_irqs+0x68/0xb0
   tasklet_action_common.isra.6+0x84/0x138
   tasklet_action+0x2c/0x38
   __do_softirq+0x120/0x324
   run_ksoftirqd+0x44/0x60
   smpboot_thread_fn+0x1ac/0x1e8
   kthread+0x134/0x138
   ret_from_fork+0x10/0x18

As we know, softirq is a asynchronous mechanism. The process of free
irqs does not take resend irq handling into account. Thus, the irq_desc
may be already freed before softirq running, and resend_irqs() does not
check the validation of irq_desc, finally, the Oops occur. Process is as
follows:

  1):
  __setup_irq
    irq_startup
      check_irq_resend  // activate softirq to handle resend irq
  2):
  irq_domain_free_irqs
    irq_free_descs
      free_desc
        call_rcu(&desc->rcu, delayed_free_desc)
  3):
  __do_softirq
    tasklet_action
      resend_irqs
        desc = irq_to_desc(irq)
        desc->handle_irq(desc)  // desc is NULL, Oops occur

Fix this by checking for a NULL pointer when get @irq_desc in
resend_irqs().

Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
Reviewed-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
---
 kernel/irq/resend.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/irq/resend.c b/kernel/irq/resend.c
index 95414ad..98c04ca5 100644
--- a/kernel/irq/resend.c
+++ b/kernel/irq/resend.c
@@ -36,6 +36,8 @@ static void resend_irqs(unsigned long arg)
 		irq = find_first_bit(irqs_resend, nr_irqs);
 		clear_bit(irq, irqs_resend);
 		desc = irq_to_desc(irq);
+		if (!desc)
+			continue;
 		local_irq_disable();
 		desc->handle_irq(desc);
 		local_irq_enable();
-- 
1.8.3.1