netdev - [PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG

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Message-Id: <20190821121720.22009-1-jakub@cloudflare.com>
Date:   Wed, 21 Aug 2019 14:17:20 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, kernel-team@...udflare.com,
        Petar Penkov <ppenkov@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: [PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG_DETACH

Call to bpf_prog_put(), with help of call_rcu(), queues an RCU-callback to
free the program once a grace period has elapsed. The callback can run
together with new RCU readers that started after the last grace period.
New RCU readers can potentially see the "old" to-be-freed or already-freed
pointer to the program object before the RCU update-side NULLs it.

Reorder the operations so that the RCU update-side resets the protected
pointer before the end of the grace period after which the program will be
freed.

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Reported-by: Lorenz Bauer <lmb@...udflare.com>
Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 net/core/flow_dissector.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 3e6fedb57bc1..2470b4b404e6 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -142,8 +142,8 @@ int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
 		mutex_unlock(&flow_dissector_mutex);
 		return -ENOENT;
 	}
-	bpf_prog_put(attached);
 	RCU_INIT_POINTER(net->flow_dissector_prog, NULL);
+	bpf_prog_put(attached);
 	mutex_unlock(&flow_dissector_mutex);
 	return 0;
 }
-- 
2.20.1