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

[Breno Leitao]

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 <leitao@xxxxxxxxxx>
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 <leitao@xxxxxxxxxx>

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();
 	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;
 }