[PATCH 3.16 66/83] genirq: Prevent NULL pointer dereference in resend_irqs()

[Ben Hutchings <ben@decadent.org.uk>]

3.16.78-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Yunfeng Ye <yeyunfeng@huawei.com>

commit eddf3e9c7c7e4d0707c68d1bb22cc6ec8aef7d4a upstream.

The following crash was observed:

  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

The reason for this is that the interrupt resend mechanism happens in soft
interrupt context, which is a asynchronous mechanism versus other
operations on interrupts. free_irq() does not take resend handling into
account. Thus, the irq descriptor might be already freed before the resend
tasklet is executed. resend_irqs() does not check the return value of the
interrupt descriptor lookup and derefences the return value
unconditionally.

  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 --> Ooops

Fix this by adding a NULL pointer check in resend_irqs() before derefencing
the irq descriptor.

Fixes: a4633adcdbc1 ("[PATCH] genirq: add genirq sw IRQ-retrigger")
Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Link: https://lkml.kernel.org/r/1630ae13-5c8e-901e-de09-e740b6a426a7@huawei.com
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 kernel/irq/resend.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/irq/resend.c
+++ b/kernel/irq/resend.c
@@ -37,6 +37,8 @@ static void resend_irqs(unsigned long ar
 		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(irq, desc);
 		local_irq_enable();