netdev - Re: [PATCH bpf] bpf: Fix a race in reuseport_array

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Message-ID: <20190927181729.7ep3pp2hiy6l5ixk@kafai-mbp.dhcp.thefacebook.com>
Date:   Fri, 27 Sep 2019 18:17:36 +0000
From:   Martin Lau <kafai@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: Fix a race in reuseport_array_free()

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?

> 
> > 
> > This patch takes the reuseport_lock in reuseport_array_free() which
> > is not the fast path.  The lock is taken inside the loop in case
> > that the bpf map is big.
> > 
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> 
> Fixes: 5dc4c4b7d4e8 ("bpf: Introduce BPF_MAP_TYPE_REUSEPORT_SOCKARRAY")
Ah...missed that.  Thanks!