netdev - Re: [PATCH] ipv4: ping: Fix potential use-after-free bug

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Date:   Sun, 18 Sep 2022 09:30:21 -0600
From:   David Ahern <dsahern@...nel.org>
To:     Liang He <windhl@....com>, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH] ipv4: ping: Fix potential use-after-free bug

On 9/16/22 4:07 AM, Liang He wrote:
> In ping_unhash(), we should move sock_put(sk) after any possible
> access point as the put function may free the object.

unhash handlers are called from sk_common_release which still has a
reference on the sock, so not really going to hit a UAF.

I do agree that it does not read correctly to 'put' a reference then
continue using the object. ie., the put should be moved to the end like
you have here. This is more of a tidiness exercise than a need to
backport to stable kernels.

> 
> Fixes: c319b4d76b9e ("net: ipv4: add IPPROTO_ICMP socket kind")
> Signed-off-by: Liang He <windhl@....com>
> ---
> 
>  I have found other places containing similar code patterns.
> 
>  net/ipv4/ping.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/ping.c b/net/ipv4/ping.c
> index b83c2bd9d722..f90c86d37ffc 100644
> --- a/net/ipv4/ping.c
> +++ b/net/ipv4/ping.c
> @@ -157,10 +157,10 @@ void ping_unhash(struct sock *sk)
>  	spin_lock(&ping_table.lock);
>  	if (sk_hashed(sk)) {
>  		hlist_nulls_del_init_rcu(&sk->sk_nulls_node);
> -		sock_put(sk);
>  		isk->inet_num = 0;
>  		isk->inet_sport = 0;
>  		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
> +		sock_put(sk);
>  	}
>  	spin_unlock(&ping_table.lock);
>  }