Subject: Re: [PATCH net v4] l2tp: Serialize access to sk_user_data with sk_callback_lock

From: Hangbin Liu <[email-masked]>
Date: Fri, 2 Dec 2022 17:50:23 +0800

On Mon, Nov 14, 2022 at 08:16:19PM +0100, Jakub Sitnicki wrote:
> sk->sk_user_data has multiple users, which are not compatible with each
> other. Writers must synchronize by grabbing the sk->sk_callback_lock.
> 
> l2tp currently fails to grab the lock when modifying the underlying tunnel
> socket fields. Fix it by adding appropriate locking.
> 
> We err on the side of safety and grab the sk_callback_lock also inside the
> sk_destruct callback overridden by l2tp, even though there should be no
> refs allowing access to the sock at the time when sk_destruct gets called.
> 
> v4:
> - serialize write to sk_user_data in l2tp sk_destruct
> 
> v3:
> - switch from sock lock to sk_callback_lock
> - document write-protection for sk_user_data
> 
> v2:
> - update Fixes to point to origin of the bug
> - use real names in Reported/Tested-by tags
> 
> Cc: Tom Parkin <[email-masked]>
> Fixes: 3557baabf280 ("[L2TP]: PPP over L2TP driver core")
> Reported-by: Haowei Yan <[email-masked]>
> Signed-off-by: Jakub Sitnicki <[email-masked]>
> ---
> 
> This took me forever. Sorry about that.
> 
>  include/net/sock.h   |  2 +-
>  net/l2tp/l2tp_core.c | 19 +++++++++++++------
>  2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 5db02546941c..e0517ecc6531 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -323,7 +323,7 @@ struct sk_filter;
>    *	@sk_tskey: counter to disambiguate concurrent tstamp requests
>    *	@sk_zckey: counter to order MSG_ZEROCOPY notifications
>    *	@sk_socket: Identd and reporting IO signals
> -  *	@sk_user_data: RPC layer private data
> +  *	@sk_user_data: RPC layer private data. Write-protected by @sk_callback_lock.
>    *	@sk_frag: cached page frag
>    *	@sk_peek_off: current peek_offset value
>    *	@sk_send_head: front of stuff to transmit
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 7499c51b1850..754fdda8a5f5 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1150,8 +1150,10 @@ static void l2tp_tunnel_destruct(struct sock *sk)
>  	}
>  
>  	/* Remove hooks into tunnel socket */
> +	write_lock_bh(&sk->sk_callback_lock);
>  	sk->sk_destruct = tunnel->old_sk_destruct;
>  	sk->sk_user_data = NULL;
> +	write_unlock_bh(&sk->sk_callback_lock);
>  
>  	/* Call the original destructor */
>  	if (sk->sk_destruct)

Hi Jakub,

I have a similar issue with vxlan driver. Similar with commit
ad6c9986bcb6 ("vxlan: Fix GRO cells race condition between receive and link
delete"). There is still a race condition on vxlan that when receive a packet
while deleting a VXLAN device. In vxlan_ecn_decapsulate(), the
vxlan_get_sk_family() call panic as sk is NULL.

 #0 [ffffa25ec6978a38] machine_kexec at ffffffff8c669757
 #1 [ffffa25ec6978a90] __crash_kexec at ffffffff8c7c0a4d
 #2 [ffffa25ec6978b58] crash_kexec at ffffffff8c7c1c48
 #3 [ffffa25ec6978b60] oops_end at ffffffff8c627f2b
 #4 [ffffa25ec6978b80] page_fault_oops at ffffffff8c678fcb
 #5 [ffffa25ec6978bd8] exc_page_fault at ffffffff8d109542
 #6 [ffffa25ec6978c00] asm_exc_page_fault at ffffffff8d200b62
    [exception RIP: vxlan_ecn_decapsulate+0x3b]
    RIP: ffffffffc1014e7b  RSP: ffffa25ec6978cb0  RFLAGS: 00010246
    RAX: 0000000000000008  RBX: ffff8aa000888000  RCX: 0000000000000000
    RDX: 000000000000000e  RSI: ffff8a9fc7ab803e  RDI: ffff8a9fd1168700
    RBP: ffff8a9fc7ab803e   R8: 0000000000700000   R9: 00000000000010ae
    R10: ffff8a9fcb748980  R11: 0000000000000000  R12: ffff8a9fd1168700
    R13: ffff8aa000888000  R14: 00000000002a0000  R15: 00000000000010ae
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #7 [ffffa25ec6978ce8] vxlan_rcv at ffffffffc10189cd [vxlan]
 #8 [ffffa25ec6978d90] udp_queue_rcv_one_skb at ffffffff8cfb6507
 #9 [ffffa25ec6978dc0] udp_unicast_rcv_skb at ffffffff8cfb6e45
#10 [ffffa25ec6978dc8] __udp4_lib_rcv at ffffffff8cfb8807
#11 [ffffa25ec6978e20] ip_protocol_deliver_rcu at ffffffff8cf76951
#12 [ffffa25ec6978e48] ip_local_deliver at ffffffff8cf76bde
#13 [ffffa25ec6978ea0] __netif_receive_skb_one_core at ffffffff8cecde9b
#14 [ffffa25ec6978ec8] process_backlog at ffffffff8cece139
#15 [ffffa25ec6978f00] __napi_poll at ffffffff8ceced1a
#16 [ffffa25ec6978f28] net_rx_action at ffffffff8cecf1f3
#17 [ffffa25ec6978fa0] __softirqentry_text_start at ffffffff8d4000ca
#18 [ffffa25ec6978ff0] do_softirq at ffffffff8c6fbdc3
--- <IRQ stack> ---

> struct socket ffff8a9fd1168700
struct socket {
  state = SS_FREE,
  type = 0,
  flags = 0,
  file = 0xffff8a9fcb748000,
  sk = 0x0,
  ops = 0x0,

So I'm wondering if we should also have locks in udp_tunnel_sock_release().
Or should we add a checking in sk state before calling vxlan_get_sk_family()?

Thanks
Hangbin