Subject: Re: [Question]: A non NULL req->sk in tcp_rtx_synack. Not a fastopen connection.

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 3 Oct 2024 21:00:20 -0700

On 10/3/24 7:02 PM, Kuniyuki Iwashima wrote:
> From: Martin KaFai Lau <[email-masked]>
> Date: Thu, 3 Oct 2024 18:14:09 -0700
>> Hi,
>>
>> We are seeing a use-after-free from a bpf prog attached to
>> trace_tcp_retransmit_synack. The program passes the req->sk to the
>> bpf_sk_storage_get_tracing kernel helper which does check for null before using it.
>>
>> fastopen is not used.
>>
>> We got a kfence report on use-after-free (pasted at the end). It is running with
>> an older 6.4 kernel and we hardly hit this in production.
>>
>>   From the upstream code, del_timer_sync() should have been done by
>> inet_csk_reqsk_queue_drop() before "req->sk = child;" is assigned in
>> inet_csk_reqsk_queue_add(). My understanding is the req->rsk_timer should have
>> been stopped before the "req->sk = child;" assignment.
> 
> There seems to be a small race window in reqsk_queue_unlink().
> 
> expire_timers() first calls detach_timer(, true), which marks the timer
> as not pending, and then calls reqsk_timer_handler().
> 
> If reqsk_queue_unlink() calls timer_pending() just before expire_timers()
> calls reqsk_timer_handler(), reqsk_queue_unlink() could miss
> del_timer_sync() ?

This seems to explain it. :)

Does it mean there is a chance that the reqsk_timer_handler() may rearm the 
timer again and I guess only a few more synack will be sent in this case and 
should be no harm?

> 
> ---8<---
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index 2c5632d4fddb..4ba47ee6c9da 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -1045,7 +1045,7 @@ static bool reqsk_queue_unlink(struct request_sock *req)
>   		found = __sk_nulls_del_node_init_rcu(sk);
>   		spin_unlock(lock);
>   	}
> -	if (timer_pending(&req->rsk_timer) && del_timer_sync(&req->rsk_timer))
> +	if (del_timer_sync(&req->rsk_timer))

It seems the reqsk_timer_handler() will also call reqsk_queue_unlink() through 
inet_csk_reqsk_queue_drop_and_put(). Not sure if the reqsk_timer_handler() can 
del_timer_sync() itself.

>   		reqsk_put(req);
>   	return found;
>   }
> ---8<---
> 
> 
>>
>> or there are cases that req->sk is not NULL in the reqsk_timer_handler()?
>>
>> BUG: KFENCE: use-after-free read in bpf_sk_storage_get_tracing+0x2e/0x1b0
>>
>> Use-after-free read at 0x00000000a891fb3a (in kfence-#1):
>> bpf_sk_storage_get_tracing+0x2e/0x1b0
>> bpf_prog_5ea3e95db6da0438_tcp_retransmit_synack+0x1d20/0x1dda
>> bpf_trace_run2+0x4c/0xc0
>> tcp_rtx_synack+0xf9/0x100
>> reqsk_timer_handler+0xda/0x3d0
>> run_timer_softirq+0x292/0x8a0
>> irq_exit_rcu+0xf5/0x320
>> sysvec_apic_timer_interrupt+0x6d/0x80
>> asm_sysvec_apic_timer_interrupt+0x16/0x20
>> intel_idle_irq+0x5a/0xa0
>> cpuidle_enter_state+0x94/0x273
>> cpu_startup_entry+0x15e/0x260
>> start_secondary+0x8a/0x90
>> secondary_startup_64_no_verify+0xfa/0xfb
>>
>> kfence-#1: 0x00000000a72cc7b6-0x00000000d97616d9, size=2376, cache=TCPv6
>>
>> allocated by task 0 on cpu 9 at 260507.901592s:
>> sk_prot_alloc+0x35/0x140
>> sk_clone_lock+0x1f/0x3f0
>> inet_csk_clone_lock+0x15/0x160
>> tcp_create_openreq_child+0x1f/0x410
>> tcp_v6_syn_recv_sock+0x1da/0x700
>> tcp_check_req+0x1fb/0x510
>> tcp_v6_rcv+0x98b/0x1420
>> ipv6_list_rcv+0x2258/0x26e0
>> napi_complete_done+0x5b1/0x2990
>> mlx5e_napi_poll+0x2ae/0x8d0
>> net_rx_action+0x13e/0x590
>> irq_exit_rcu+0xf5/0x320
>> common_interrupt+0x80/0x90
>> asm_common_interrupt+0x22/0x40
>> cpuidle_enter_state+0xfb/0x273
>> cpu_startup_entry+0x15e/0x260
>> start_secondary+0x8a/0x90
>> secondary_startup_64_no_verify+0xfa/0xfb
>>
>> freed by task 0 on cpu 9 at 260507.927527s:
>> rcu_core_si+0x4ff/0xf10
>> irq_exit_rcu+0xf5/0x320
>> sysvec_apic_timer_interrupt+0x6d/0x80
>> asm_sysvec_apic_timer_interrupt+0x16/0x20
>> cpuidle_enter_state+0xfb/0x273
>> cpu_startup_entry+0x15e/0x260
>> start_secondary+0x8a/0x90
>> secondary_startup_64_no_verify+0xfa/0xfb
>>
>> Thanks,
>> Martin