linux-kernel - Re: [PATCH v4 1/2] drivers: net: prevent tun_build

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Date:   Tue, 1 Aug 2023 19:07:39 -0600
From:   David Ahern <dsahern@...il.com>
To:     Andrew Kanner <andrew.kanner@...il.com>, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        jasowang@...hat.com, netdev@...r.kernel.org, brouer@...hat.com,
        jbrouer@...hat.com, john.fastabend@...il.com,
        linux-kernel@...r.kernel.org
Cc:     linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+f817490f5bd20541b90a@...kaller.appspotmail.com
Subject: Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed
 the packet size limit

On 8/1/23 4:07 PM, Andrew Kanner wrote:
> @@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>  	if (zerocopy)
>  		return false;
>  
> -	if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> +	rcu_read_lock();
> +	xdp_prog = rcu_dereference(tun->xdp_prog);
> +	if (xdp_prog)
> +		pad += XDP_PACKET_HEADROOM;
> +	rcu_read_unlock();


since you do not care about the actual xdp_prog (only that one is set) I
believe you can use rcu_access_pointer here.