Message-ID: <53664FF4-A917-46FE-AEA7-45F31CE1CD88@oracle.com>
Date: Sun, 14 May 2023 18:29:46 +0000
From: Chuck Lever III <chuck.lever@...cle.com>
To: Ding Hui <dinghui@...gfor.com.cn>
CC: "jlayton@...nel.org" <jlayton@...nel.org>,
        "trond.myklebust@...merspace.com" <trond.myklebust@...merspace.com>,
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        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
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	<netdev@...r.kernel.org>,
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Subject: Re: [RFC PATCH] SUNRPC: Fix UAF in svc_tcp_listen_data_ready()

[ Removing the stale address for Bruce from the Cc, as he no longer
  works at Red Hat. ]


> On May 7, 2023, at 9:32 PM, Ding Hui <dinghui@...gfor.com.cn> wrote:
> 
> On 2023/5/7 23:26, Chuck Lever III wrote:
>>> On May 7, 2023, at 5:11 AM, Ding Hui <dinghui@...gfor.com.cn> 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.
> 
> I checked the vmcore, found that sk_odata points to sock_def_readable(),
> and the sk_wq of newsock is NULL, which be assigned by sk_clone_lock()
> unconditionally.
> 
> Calling sk_odata() for the newsock maybe do not wake up any sleepers.
> 
>> 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.
> 
> Agree, but I don't have a good way for now.
> 
>>> 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

I haven't been able to reproduce a crash with this snippet. But
I've done some archaeology to understand the problem better.

I found that svc_tcp_listen_data_ready is actually invoked /three/
times: once for the listener socket, and /twice/ for the child.
The big comment, which pre-dates the git era, appears to be
somewhat stale; or perhaps it's the specifics of this particular
test that triggers the third call.

I reviewed several other tcp_listen_data_ready callbacks. They
generally do not do anything at all with non-listener sockets,
suggesting that approach would likely be safe for NFSD.

Prior to commit 939bb7ef901b ("[PATCH] Code cleanups in calbacks
in svcsock"), this data_ready callback was a complete no-op for
non-listener sockets as well. That commit is described as only
a clean-up, but it indeed changes the logic.

I also note that most other data_ready callbacks take the
sk_callback_lock, and svc_tcp_listen_data_ready does not. Not
clear to me whether svc_tcp_listen_data_ready should be taking
that lock too.

The upshot is that I think it would be reasonable to simply do
nothing in svc_tcp_listen_data_ready() if state != TCP_LISTEN.


>>> 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@...gfor.com.cn>
>>> ---
>>> 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
> 
> -- 
> Thanks,
> - Ding Hui
> 

--
Chuck Lever

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