We need to sync packet rx again after flushing the queue entries.
Otherwise, the following race could happen:
cpu1: nf_unregister_hook(H) called, H unliked from lists, calls
synchronize_net() to wait for packet rx completion.
Problem is that while no new nf_queue_entry structs that use H can be
allocated, another CPU might receive a verdict from userspace just before
cpu1 calls nf_queue_nf_hook_drop to remove this entry:
cpu2: receive verdict from userspace, lock queue
cpu2: unlink nf_queue_entry struct E, which references H, from queue list
cpu1: calls nf_queue_nf_hook_drop, blocks on queue spinlock
cpu2: unlock queue
cpu1: nf_queue_nf_hook_drop drops affected queue entries
cpu2: call nf_reinject for E
cpu1: kfree(H)
cpu2: potential use-after-free for H
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Fixes: 085db2c04557 ("netfilter: Per network namespace netfilter hooks.")
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
---
Found during code review, no actual panic was observed.
diff --git a/net/netfilter/core.c b/net/netfilter/core.c
index 8e47f81..21a0856 100644
--- a/net/netfilter/core.c
+++ b/net/netfilter/core.c
@@ -152,6 +152,8 @@ void nf_unregister_net_hook(struct net *net, const struct nf_hook_ops *reg)
#endif
synchronize_net();
nf_queue_nf_hook_drop(net, &entry->ops);
+ /* other cpu might still process nfqueue verdict that used reg */
+ synchronize_net();
kfree(entry);
}
EXPORT_SYMBOL(nf_unregister_net_hook);
--
2.0.5
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