Re: [RFC PATCH] SUNRPC: Fix UAF in svc_tcp_listen_data_ready()

[Chuck Lever III]

> On May 7, 2023, at 5:11 AM, Ding Hui <dinghui@xxxxxxxxxxxxxx> wrote:
> 
> After the listener svc_sock freed, and before invoking svc_tcp_accept()
> for the established child sock, there is a window that the newsock
> retaining a freed listener svc_sock in sk_user_data which cloning from
> parent. In the race windows if data is received on the newsock, we will
> observe use-after-free report in svc_tcp_listen_data_ready().

My thought is that not calling sk_odata() for the newsock
could potentially result in missing a data_ready event,
resulting in a hung client on that socket.

IMO the preferred approach is to ensure that svsk is always
safe to dereference in tcp_listen_data_ready. I haven't yet
thought carefully about how to do that.


> Reproduce by two tasks:
> 
> 1. while :; do rpc.nfsd 0 ; rpc.nfsd; done
> 2. while :; do echo "" | ncat -4 127.0.0.1 2049 ; done
> 
> KASAN report:
> 
>  ==================================================================
>  BUG: KASAN: slab-use-after-free in svc_tcp_listen_data_ready+0x1cf/0x1f0 [sunrpc]
>  Read of size 8 at addr ffff888139d96228 by task nc/102553
>  CPU: 7 PID: 102553 Comm: nc Not tainted 6.3.0+ #18
>  Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
>  Call Trace:
>   <IRQ>
>   dump_stack_lvl+0x33/0x50
>   print_address_description.constprop.0+0x27/0x310
>   print_report+0x3e/0x70
>   kasan_report+0xae/0xe0
>   svc_tcp_listen_data_ready+0x1cf/0x1f0 [sunrpc]
>   tcp_data_queue+0x9f4/0x20e0
>   tcp_rcv_established+0x666/0x1f60
>   tcp_v4_do_rcv+0x51c/0x850
>   tcp_v4_rcv+0x23fc/0x2e80
>   ip_protocol_deliver_rcu+0x62/0x300
>   ip_local_deliver_finish+0x267/0x350
>   ip_local_deliver+0x18b/0x2d0
>   ip_rcv+0x2fb/0x370
>   __netif_receive_skb_one_core+0x166/0x1b0
>   process_backlog+0x24c/0x5e0
>   __napi_poll+0xa2/0x500
>   net_rx_action+0x854/0xc90
>   __do_softirq+0x1bb/0x5de
>   do_softirq+0xcb/0x100
>   </IRQ>
>   <TASK>
>   ...
>   </TASK>
> 
>  Allocated by task 102371:
>   kasan_save_stack+0x1e/0x40
>   kasan_set_track+0x21/0x30
>   __kasan_kmalloc+0x7b/0x90
>   svc_setup_socket+0x52/0x4f0 [sunrpc]
>   svc_addsock+0x20d/0x400 [sunrpc]
>   __write_ports_addfd+0x209/0x390 [nfsd]
>   write_ports+0x239/0x2c0 [nfsd]
>   nfsctl_transaction_write+0xac/0x110 [nfsd]
>   vfs_write+0x1c3/0xae0
>   ksys_write+0xed/0x1c0
>   do_syscall_64+0x38/0x90
>   entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
>  Freed by task 102551:
>   kasan_save_stack+0x1e/0x40
>   kasan_set_track+0x21/0x30
>   kasan_save_free_info+0x2a/0x50
>   __kasan_slab_free+0x106/0x190
>   __kmem_cache_free+0x133/0x270
>   svc_xprt_free+0x1e2/0x350 [sunrpc]
>   svc_xprt_destroy_all+0x25a/0x440 [sunrpc]
>   nfsd_put+0x125/0x240 [nfsd]
>   nfsd_svc+0x2cb/0x3c0 [nfsd]
>   write_threads+0x1ac/0x2a0 [nfsd]
>   nfsctl_transaction_write+0xac/0x110 [nfsd]
>   vfs_write+0x1c3/0xae0
>   ksys_write+0xed/0x1c0
>   do_syscall_64+0x38/0x90
>   entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
> In this RFC patch, I try to fix the UAF by skipping dereferencing
> svsk for all child socket in svc_tcp_listen_data_ready(), it is
> easy to backport for stable.
> 
> However I'm not sure if there are other potential risks in the race
> window, so I thought another fix which depends on SK_USER_DATA_NOCOPY
> introduced in commit f1ff5ce2cd5e ("net, sk_msg: Clear sk_user_data
> pointer on clone if tagged").
> 
> Saving svsk into sk_user_data with SK_USER_DATA_NOCOPY tag in
> svc_setup_socket() like this:
> 
>  __rcu_assign_sk_user_data_with_flags(inet, svsk, SK_USER_DATA_NOCOPY);
> 
> Obtaining svsk in callbacks like this:
> 
>  struct svc_sock *svsk = rcu_dereference_sk_user_data(sk);
> 
> This will avoid copying sk_user_data for sunrpc svc_sock in
> sk_clone_lock(), so the sk_user_data of child sock before accepted
> will be NULL.
> 
> Appreciate any comment and suggestion, thanks.
> 
> Fixes: fa9251afc33c ("SUNRPC: Call the default socket callbacks instead of open coding")
> Signed-off-by: Ding Hui <dinghui@xxxxxxxxxxxxxx>
> ---
> net/sunrpc/svcsock.c | 23 +++++++++++------------
> 1 file changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index a51c9b989d58..9aca6e1e78e4 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -825,12 +825,6 @@ static void svc_tcp_listen_data_ready(struct sock *sk)
> 
> trace_sk_data_ready(sk);
> 
> - if (svsk) {
> - /* Refer to svc_setup_socket() for details. */
> - rmb();
> - svsk->sk_odata(sk);
> - }
> -
> /*
> * This callback may called twice when a new connection
> * is established as a child socket inherits everything
> @@ -839,13 +833,18 @@ static void svc_tcp_listen_data_ready(struct sock *sk)
> *    when one of child sockets become ESTABLISHED.
> * 2) data_ready method of the child socket may be called
> *    when it receives data before the socket is accepted.
> - * In case of 2, we should ignore it silently.
> + * In case of 2, we should ignore it silently and DO NOT
> + * dereference svsk.
> */
> - if (sk->sk_state == TCP_LISTEN) {
> - if (svsk) {
> - set_bit(XPT_CONN, &svsk->sk_xprt.xpt_flags);
> - svc_xprt_enqueue(&svsk->sk_xprt);
> - }
> + if (sk->sk_state != TCP_LISTEN)
> + return;
> +
> + if (svsk) {
> + /* Refer to svc_setup_socket() for details. */
> + rmb();
> + svsk->sk_odata(sk);
> + set_bit(XPT_CONN, &svsk->sk_xprt.xpt_flags);
> + svc_xprt_enqueue(&svsk->sk_xprt);
> }
> }
> 
> -- 
> 2.17.1
> 

--
Chuck Lever