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@cloudflare.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: Petar Penkov <ppenkov@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 3e6fedb57bc100e8ce002deddbade59371a73065..2470b4b404e62e625b17714f27fa7aba58eef9c5 100644 (file)
--- 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;
 }