[PATCH v2 1/1] xfrm : lock input tasklet skb queue

[Tom Rix <trix@redhat.com>]

On PREEMPT_RT_FULL while running netperf, a corruption
of the skb queue causes an oops.

This appears to be caused by a race condition here
        __skb_queue_tail(&trans->queue, skb);
        tasklet_schedule(&trans->tasklet);
Where the queue is changed before the tasklet is locked by
tasklet_schedule.

The fix is to use the skb queue lock.

This is the original work of Joerg Vehlow <joerg.vehlow@aox-tech.de>
https://lkml.org/lkml/2019/9/9/111
  xfrm_input: Protect queue with lock

  During the skb_queue_splice_init the tasklet could have been preempted
  and __skb_queue_tail called, which led to an inconsistent queue.

ifdefs for CONFIG_PREEMPT_RT_FULL added to reduce runtime effects
on the normal kernel.

Signed-off-by: Tom Rix <trix@redhat.com>
---
 net/xfrm/xfrm_input.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
index 9b599ed66d97..decd515f84cf 100644
--- a/net/xfrm/xfrm_input.c
+++ b/net/xfrm/xfrm_input.c
@@ -755,13 +755,21 @@ EXPORT_SYMBOL(xfrm_input_resume);

 static void xfrm_trans_reinject(unsigned long data)
 {
+#ifdef CONFIG_PREEMPT_RT_FULL
+    unsigned long flags;
+#endif
     struct xfrm_trans_tasklet *trans = (void *)data;
     struct sk_buff_head queue;
     struct sk_buff *skb;

     __skb_queue_head_init(&queue);
+#ifdef CONFIG_PREEMPT_RT_FULL
+    spin_lock_irqsave(&trans->queue.lock, flags);
+#endif
     skb_queue_splice_init(&trans->queue, &queue);
-
+#ifdef CONFIG_PREEMPT_RT_FULL
+    spin_unlock_irqrestore(&trans->queue.lock, flags);
+#endif
     while ((skb = __skb_dequeue(&queue)))
         XFRM_TRANS_SKB_CB(skb)->finish(dev_net(skb->dev), NULL, skb);
 }
@@ -778,7 +786,11 @@ int xfrm_trans_queue(struct sk_buff *skb,
         return -ENOBUFS;

     XFRM_TRANS_SKB_CB(skb)->finish = finish;
+#ifdef CONFIG_PREEMPT_RT_FULL
+    skb_queue_tail(&trans->queue, skb);
+#else
     __skb_queue_tail(&trans->queue, skb);
+#endif
     tasklet_schedule(&trans->tasklet);
     return 0;
 }
@@ -798,7 +810,11 @@ void __init xfrm_input_init(void)
         struct xfrm_trans_tasklet *trans;

         trans = &per_cpu(xfrm_trans_tasklet, i);
+#ifdef CONFIG_PREEMPT_RT_FULL
+        skb_queue_head_init(&trans->queue);
+#else
         __skb_queue_head_init(&trans->queue);
+#endif
         tasklet_init(&trans->tasklet, xfrm_trans_reinject,
                  (unsigned long)trans);
     }
-- 
2.23.0