netdev - [PATCH net] netkit: Ensure current->bpf_net_context is set in netkit

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Message-ID: <20240914184616.2916445-1-jrife@google.com>
Date: Sat, 14 Sep 2024 18:46:12 +0000
From: Jordan Rife <jrife@...gle.com>
To: Daniel Borkmann <daniel@...earbox.net>, Nikolay Aleksandrov <razor@...ckwall.org>, netdev@...r.kernel.org
Cc: Jordan Rife <jrife@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	"Toke Høiland-Jørgensen" <toke@...hat.com>, bpf@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH net] netkit: Ensure current->bpf_net_context is set in netkit_xmit()

When operating Cilium in netkit mode with BPF-based host routing, calls
to bpf_redirect() cause a kernel panic.

[   52.247646] BUG: kernel NULL pointer dereference, address: 0000000000000038
...
[   52.247727] RIP: 0010:bpf_redirect+0x18/0x80
...
[   52.247986] Call Trace:
[   52.247990]  <TASK>
[   52.248002]  ? show_regs+0x6c/0x80
[   52.248024]  ? __die+0x24/0x80
[   52.248029]  ? page_fault_oops+0x155/0x570
[   52.248047]  ? fib_rules_lookup+0x112/0x270
[   52.248056]  ? do_user_addr_fault+0x4b2/0x870
[   52.248063]  ? exc_page_fault+0x82/0x1b0
[   52.248090]  ? asm_exc_page_fault+0x27/0x30
[   52.248103]  ? bpf_redirect+0x18/0x80
[   52.248109]  bpf_prog_f0698aabaf44c832_tail_handle_ipv4+0x173f/0x2707
[   52.248119]  ? sbitmap_find_bit+0xe3/0x270
[   52.248129]  netkit_xmit+0x177/0x3c0
[   52.248139]  dev_hard_start_xmit+0x62/0x1d0
[   52.248149]  __dev_queue_xmit+0x241/0xf30
[   52.248155]  ? alloc_skb_with_frags+0x60/0x280
[   52.248164]  ? __check_object_size+0x2a2/0x310
[   52.248173]  ? ip_generic_getfrag+0x63/0x110
[   52.248181]  ip_finish_output2+0x2cf/0x560
[   52.248187]  __ip_finish_output+0xb6/0x180
[   52.248193]  ip_finish_output+0x29/0x120
[   52.248198]  ip_output+0x5f/0x100
[   52.248204]  ? __pfx_ip_finish_output+0x10/0x10
[   52.248210]  ip_send_skb+0x98/0xb0
[   52.248215]  udp_send_skb+0x146/0x370

Setting a breakpoint inside bpf_net_ctx_get_ri() confirms that
current->bpf_net_context is NULL right before the panic.

(gdb) p $lx_current().bpf_net_context
$4 = (struct bpf_net_context *) 0x0 <fixed_percpu_data>
(gdb) disassemble bpf_redirect
Dump of assembler code for function bpf_redirect:
   0xffffffff81f085e0 <+0>:	nopl   0x0(%rax,%rax,1)
   0xffffffff81f085e5 <+5>:	mov    %gs:0x7e12d593(%rip),%rax
   0xffffffff81f085ed <+13>:	push   %rbp
   0xffffffff81f085ee <+14>:	mov    0x23d0(%rax),%rax
=> 0xffffffff81f085f5 <+21>:	mov    %rsp,%rbp
   0xffffffff81f085f8 <+24>:	mov    0x38(%rax),%edx
...
(gdb) continue
Continuing.

Thread 1 hit Breakpoint 1, panic ...
288	{
(gdb)

commit 401cb7dae813 ("net: Reference bpf_redirect_info via task_struct
on PREEMPT_RT.") recently moved bpf_redirect_info into bpf_net_context,
a new member of task_struct. Currently, current->bpf_net_context is set
and then cleared inside sch_handle_egress() where tcx_run() and tc_run()
execute, but it looks like netkit_xmit() was missed leaving
current->bpf_net_context uninitialized when it runs. This patch ensures
that current->bpf_net_context is initialized while running
netkit_xmit().

Signed-off-by: Jordan Rife <jrife@...gle.com>
Fixes: 401cb7dae813 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Cc: stable@...r.kernel.org
---
 drivers/net/netkit.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/netkit.c b/drivers/net/netkit.c
index d0036a856039..92ac0cb5a327 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;
@@ -73,6 +74,7 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
 	int len = skb->len;
 
 	rcu_read_lock();
+	bpf_net_ctx = bpf_net_ctx_set(&__bpf_net_ctx);
 	peer = rcu_dereference(nk->peer);
 	if (unlikely(!peer || !(peer->flags & IFF_UP) ||
 		     !pskb_may_pull(skb, ETH_HLEN) ||
@@ -109,6 +111,7 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
 		ret_dev = NET_XMIT_DROP;
 		break;
 	}
+	bpf_net_ctx_clear(bpf_net_ctx);
 	rcu_read_unlock();
 	return ret_dev;
 }
-- 
2.46.0.662.g92d0881bb0-goog