Re: [PATCH bpf] bpf: Fix a race in reuseport_array_free()

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

On 9/27/19 11:17 AM, Martin Lau wrote:
> On Fri, Sep 27, 2019 at 10:24:49AM -0700, Eric Dumazet wrote:
>>
>>
>> On 9/27/19 9:52 AM, Martin KaFai Lau wrote:
>>> In reuseport_array_free(), the rcu_read_lock() cannot ensure sk is still
>>> valid.  It is because bpf_sk_reuseport_detach() can be called from
>>> __sk_destruct() which is invoked through call_rcu(..., __sk_destruct).
>>
>> We could question why reuseport_detach_sock(sk) is called from __sk_destruct()
>> (after the rcu grace period) instead of sk_destruct() ?
> Agree.  It is another way to fix it.
> 
> In this patch, I chose to avoid the need to single out a special treatment for
> reuseport_detach_sock() in sk_destruct().
> 
> I am happy either way.  What do you think?

It seems that since we call reuseport_detach_sock() after the rcu grace period,
another cpu could catch the sk pointer in reuse->socks[] array and use
it right before our cpu frees the socket.

RCU rules are not properly applied here I think.

The rules for deletion are :

1) unpublish object from various lists/arrays/hashes.
2) rcu_grace_period
3) free the object.

If we fix the unpublish (we need to anyway to make the data path safe),
then your patch is not needed ?

What about (totally untested, might be horribly wrong)

diff --git a/net/core/sock.c b/net/core/sock.c
index 07863edbe6fc4842e47ebebf00bc21bc406d9264..d31a4b094797f73ef89110c954aa0a164879362d 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1700,8 +1700,6 @@ static void __sk_destruct(struct rcu_head *head)
                sk_filter_uncharge(sk, filter);
                RCU_INIT_POINTER(sk->sk_filter, NULL);
        }
-       if (rcu_access_pointer(sk->sk_reuseport_cb))
-               reuseport_detach_sock(sk);
 
        sock_disable_timestamp(sk, SK_FLAGS_TIMESTAMP);
 
@@ -1728,7 +1726,13 @@ static void __sk_destruct(struct rcu_head *head)
 
 void sk_destruct(struct sock *sk)
 {
-       if (sock_flag(sk, SOCK_RCU_FREE))
+       bool use_call_rcu = sock_flag(sk, SOCK_RCU_FREE);
+
+       if (rcu_access_pointer(sk->sk_reuseport_cb)) {
+               reuseport_detach_sock(sk);
+               use_call_rcu = true;
+       }
+       if (use_call_rcu)
                call_rcu(&sk->sk_rcu, __sk_destruct);
        else
                __sk_destruct(&sk->sk_rcu);