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

[Ding Hui <dinghui@sangfor.com.cn>]

On 2023/5/7 23:26, Chuck Lever III wrote:
> 
> 
>> On May 7, 2023, at 5:11 AM, Ding Hui <dinghui@sangfor.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
>>
>> 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@sangfor.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