netdev - Re: [PATCH net] Revert "tcp: switch tcp_fastopen key generation to net_get_random

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Date:	Thu, 18 Jun 2015 11:32:33 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Christoph Paasch <cpaasch@...le.com>, netdev@...r.kernel.org
Cc:	David Miller <davem@...emloft.net>
Subject: Re: [PATCH net] Revert "tcp: switch tcp_fastopen key generation to
 net_get_random_once"

Hello Christoph,

On Wed, 2015-06-17 at 17:28 -0700, Christoph Paasch wrote:
> This reverts commit 222e83d2e0aecb6a5e8d42b1a8d51332a1eba960.
> 
> tcp_fastopen_reset_cipher really cannot be called from interrupt
> context. It allocates the tcp_fastopen_context with GFP_KERNEL and
> calls crypto_alloc_cipher, which allocates all kind of stuff with
> GFP_KERNEL.
> 
> Thus, we might sleep when the key-generation is triggered by an
> incoming TFO cookie-request which would then happen in interrupt-
> context, as shown by enabling CONFIG_DEBUG_ATOMIC_SLEEP:
> 
> [   36.001813] BUG: sleeping function called from invalid context at 
> mm/slub.c:1266
> [   36.003624] in_atomic(): 1, irqs_disabled(): 0, pid: 1016, name: 
> packetdrill
> [   36.004859] CPU: 1 PID: 1016 Comm: packetdrill Not tainted 4.1.0-rc7 #14
> [   36.006085] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel
> -1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
> [   36.008250]  00000000000004f2 ffff88007f8838a8 ffffffff8171d53a 
> ffff880075a084a8
> [   36.009630]  ffff880075a08000 ffff88007f8838c8 ffffffff810967d3 
> ffff88007f883928
> [   36.011076]  0000000000000000 ffff88007f8838f8 ffffffff81096892 
> ffff88007f89be00
> [   36.012494] Call Trace:
> [   36.012953]  <IRQ>  [<ffffffff8171d53a>] dump_stack+0x4f/0x6d
> [   36.014085]  [<ffffffff810967d3>] ___might_sleep+0x103/0x170
> [   36.015117]  [<ffffffff81096892>] __might_sleep+0x52/0x90
> [   36.016117]  [<ffffffff8118e887>] kmem_cache_alloc_trace+0x47/0x190
> [   36.017266]  [<ffffffff81680d82>] ? tcp_fastopen_reset_cipher+0x42/0x130
> [   36.018485]  [<ffffffff81680d82>] tcp_fastopen_reset_cipher+0x42/0x130
> [   36.019679]  [<ffffffff81680f01>] tcp_fastopen_init_key_once+0x61/0x70
> [   36.020884]  [<ffffffff81680f2c>] __tcp_fastopen_cookie_gen+0x1c/0x60
> [   36.022058]  [<ffffffff816814ff>] tcp_try_fastopen+0x58f/0x730
> [   36.023118]  [<ffffffff81671788>] tcp_conn_request+0x3e8/0x7b0
> [   36.024185]  [<ffffffff810e3872>] ? __module_text_address+0x12/0x60
> [   36.025327]  [<ffffffff8167b2e1>] tcp_v4_conn_request+0x51/0x60
> [   36.026410]  [<ffffffff816727e0>] tcp_rcv_state_process+0x190/0xda0
> [   36.027556]  [<ffffffff81661f97>] ? __inet_lookup_established+0x47/0x170
> [   36.028784]  [<ffffffff8167c2ad>] tcp_v4_do_rcv+0x16d/0x3d0
> [   36.029832]  [<ffffffff812e6806>] ? security_sock_rcv_skb+0x16/0x20
> [   36.030936]  [<ffffffff8167cc8a>] tcp_v4_rcv+0x77a/0x7b0
> [   36.031875]  [<ffffffff816af8c3>] ? iptable_filter_hook+0x33/0x70
> [   36.032953]  [<ffffffff81657d22>] ip_local_deliver_finish+0x92/0x1f0
> [   36.034065]  [<ffffffff81657f1a>] ip_local_deliver+0x9a/0xb0
> [   36.035069]  [<ffffffff81657c90>] ? ip_rcv+0x3d0/0x3d0
> [   36.035963]  [<ffffffff81657569>] ip_rcv_finish+0x119/0x330
> [   36.036950]  [<ffffffff81657ba7>] ip_rcv+0x2e7/0x3d0
> [   36.037847]  [<ffffffff81610652>] __netif_receive_skb_core+0x552/0x930
> [   36.038994]  [<ffffffff81610a57>] __netif_receive_skb+0x27/0x70
> [   36.040033]  [<ffffffff81610b72>] process_backlog+0xd2/0x1f0
> [   36.041025]  [<ffffffff81611482>] net_rx_action+0x122/0x310
> [   36.042007]  [<ffffffff81076743>] __do_softirq+0x103/0x2f0
> [   36.042978]  [<ffffffff81723e3c>] do_softirq_own_stack+0x1c/0x30
> 
> There does not seem to be a better way to handle this. We could try
> to make the call to kmalloc and crypto_alloc_cipher during bootup, and
> then generate the random value only on-the-fly (when the first TFO-SYN
> comes in) with net_get_random_once in order to have the better entropy
> that comes with doing the late initialisation of the random value. But
> that's probably net-next material.

can't we simply move the net_get_random_once to the TCP_FASTOPEN setsockopt and
sendmsg(MSG_FASTOPEN) path, so those allocations still happen in process context
but we still defer the extraction of entropy as long as posible?

Thanks,
Hannes

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