Subject: Re: [PATCH net-net] tun: Assign missing bpf_net_context.

From: Vadim Fedorenko <[email-masked]>
Date: Thu, 12 Sep 2024 14:32:55 +0100

On 12/09/2024 14:17, Breno Leitao wrote:
> Hello Sabastian,
> 
> Thanks for the quick reply!
> 
> On Thu, Sep 12, 2024 at 02:28:47PM +0200, Sebastian Andrzej Siewior wrote:
>> On 2024-09-12 05:06:36 [-0700], Breno Leitao wrote:
>>> Hello Sebastian, Jakub,
>> Hi,
>>
>>> I've seen some crashes in 6.11-rc7 that seems related to 401cb7dae8130
>>> ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.").
>>>
>>> Basically bpf_net_context is NULL, and it is being dereferenced by
>>> bpf_net_ctx->ri.kern_flags (offset 0x38) in the following code.
>>>
>>> 	static inline struct bpf_redirect_info *bpf_net_ctx_get_ri(void)
>>> 	{
>>> 		struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
>>> 		if (!(bpf_net_ctx->ri.kern_flags & BPF_RI_F_RI_INIT)) {
>>>
>>> That said, it means that bpf_net_ctx_get() is returning NULL.
>>>
>>> This stack is coming from the bpf function bpf_redirect()
>>> 	BPF_CALL_2(bpf_redirect, u32, ifindex, u64, flags)
>>> 	{
>>> 	      struct bpf_redirect_info *ri = bpf_net_ctx_get_ri();
>>>
>>>
>>> Since I don't think there is XDP involved, I wondering if we need some
>>> preotection before calling bpf_redirect()
>>
>> This origins in netkit_xmit(). If my memory serves me, then Daniel told
>> me that netkit is not doing any redirect and therefore does not need
>> "this". This must have been during one of the first "designs"/ versions.
> 
> Right, I've seen several crashes related to this, and in all of them it
> is through netkit_xmit() -> netkit_run() ->  bpf_prog_run()
> 
>> If you are saying, that this is possible then something must be done.
>> Either assign a context or reject the bpf program.
> 
> If we want to assign a context, do you meant something like the
> following?
> 
> Author: Breno Leitao <[email-masked]>
> Date:   Thu Sep 12 06:11:28 2024 -0700
> 
>      netkit: Assign missing bpf_net_context.
>      
>      During the introduction of struct bpf_net_context handling for
>      XDP-redirect, the netkit driver has been missed.
>      
>      Set the bpf_net_context before invoking netkit_xmit() program within the
>      netkit driver.
>      
>      Fixes: 401cb7dae8130 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
>      Signed-off-by: Breno Leitao <[email-masked]>
> 
> diff --git a/drivers/net/netkit.c b/drivers/net/netkit.c
> index 79232f5cc088..f8af57b7a1e8 100644
> --- a/drivers/net/netkit.c
> +++ b/drivers/net/netkit.c
> @@ -65,6 +65,7 @@ static struct netkit *netkit_priv(const struct net_device *dev)
>   
>   static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
>   {
> +	struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx;
>   	struct netkit *nk = netkit_priv(dev);
>   	enum netkit_action ret = READ_ONCE(nk->policy);
>   	netdev_tx_t ret_dev = NET_XMIT_SUCCESS;
> @@ -72,6 +73,7 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
>   	struct net_device *peer;
>   	int len = skb->len;
>   
> +	bpf_net_ctx = bpf_net_ctx_set(&__bpf_net_ctx);
>   	rcu_read_lock();

Hi Breno,

looks like bpf_net_ctx should be set under rcu read lock...

>   	peer = rcu_dereference(nk->peer);
>   	if (unlikely(!peer || !(peer->flags & IFF_UP) ||
> @@ -110,6 +112,7 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
>   		break;
>   	}
>   	rcu_read_unlock();
> +	bpf_net_ctx_clear(bpf_net_ctx);
>   	return ret_dev;
>   }