Re: RDMA/addr: NULL dereference in process_one_req

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Sep 22, 2020 at 06:13:48PM +0300, Dan Aloni wrote:
> The Oops below [1], is quite rare, and occurs after awhile when kernel
> code repeatedly tries to resolve addresses. According to my analysis the
> work item is executed twice, and in the second time a NULL value of
> `req->callback` triggers this Oops.

Hum I think the race is rdma_addr_cancel(), process_one_req() and 
netevent_callback() running concurrently

It is very narrow but it looks like netevent_callback() could cause
the work to become running such that rdma_addr_cancel() has already
done the list_del_init() which causes the cancel_delayed_work() to be
skipped, and the work re-run before rdma_addr_cancel() hits its
cancel_work_sync()

Please try this:

From fac94acc7a6fb4d78ddd06c51674110937442d15 Mon Sep 17 00:00:00 2001
From: Jason Gunthorpe <jgg@nvidia.com>
Date: Tue, 22 Sep 2020 13:54:17 -0300
Subject: [PATCH] RDMA/addr: Fix race with
 netevent_callback()/rdma_addr_cancel()

This three thread race can result in the work being run once the callback
becomes NULL:

       CPU1                 CPU2                   CPU3
 netevent_callback()
                     process_one_req()       rdma_addr_cancel()
                      [..]
     spin_lock_bh()
  	set_timeout()
     spin_unlock_bh()

						spin_lock_bh()
						list_del_init(&req->list);
						spin_unlock_bh()

		     req->callback = NULL
		     spin_lock_bh()
		       if (!list_empty(&req->list))
                         // Skipped!
		         // cancel_delayed_work(&req->work);
		     spin_unlock_bh()

		    process_one_req() // again
		     req->callback() // BOOM
						cancel_delayed_work_sync()

The solution is to always cancel the work once it is completed so any
inbetween set_timeout() does not result in it running again.

Fixes: 44e75052bc2a ("RDMA/rdma_cm: Make rdma_addr_cancel into a fence")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/infiniband/core/addr.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/infiniband/core/addr.c b/drivers/infiniband/core/addr.c
index 3a98439bba832b..0abce004a9591e 100644
--- a/drivers/infiniband/core/addr.c
+++ b/drivers/infiniband/core/addr.c
@@ -647,13 +647,12 @@ static void process_one_req(struct work_struct *_work)
 	req->callback = NULL;
 
 	spin_lock_bh(&lock);
+	/*
+	 * Although the work will normally have been canceled by the workqueue,
+	 * it can still be requeued as long as it is on the req_list.
+	 */
+	cancel_delayed_work(&req->work);
 	if (!list_empty(&req->list)) {
-		/*
-		 * Although the work will normally have been canceled by the
-		 * workqueue, it can still be requeued as long as it is on the
-		 * req_list.
-		 */
-		cancel_delayed_work(&req->work);
 		list_del_init(&req->list);
 		kfree(req);
 	}
-- 
2.28.0