Date:   Mon, 16 Nov 2020 12:54:42 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Alexander Lobakin <alobakin@...me>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        0day robot <lkp@...el.com>, lkp@...ts.01.org,
        kernel test robot <oliver.sang@...el.com>
Subject: Re: [net] 0b726f6b31: BUG:unable_to_handle_page_fault_for_address

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Fri, 13 Nov 2020 10:51:36 -0500

Hi!

> On Fri, Nov 13, 2020 at 7:00 AM kernel test robot <oliver.sang@...el.com> wrote:
>>
>>
>> Greeting,
>>
>> FYI, we noticed the following commit (built with gcc-9):
>>
>> commit: 0b726f6b318a07644b6c2388e6e44406740f4754 ("[PATCH v3 net] net: udp: fix Fast/frag0 UDP GRO")
>> url: https://github.com/0day-ci/linux/commits/Alexander-Lobakin/net-udp-fix-Fast-frag0-UDP-GRO/20201110-052215
>> base: https://git.kernel.org/cgit/linux/kernel/git/davem/net.git 4e0396c59559264442963b349ab71f66e471f84d
>>
>> in testcase: apachebench
>> version:
>> with following parameters:
>>
>>         runtime: 300s
>>         concurrency: 2000
>>         cluster: cs-localhost
>>         cpufreq_governor: performance
>>         ucode: 0x7000019
>>
>> test-description: apachebench is a tool for benchmarking your Apache Hypertext Transfer Protocol (HTTP) server.
>> test-url: https://httpd.apache.org/docs/2.4/programs/ab.html
>>
>>
>> on test machine: 16 threads Intel(R) Xeon(R) CPU D-1541 @ 2.10GHz with 48G memory
>>
>> caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
>>
>>
>> If you fix the issue, kindly add following tag
>> Reported-by: kernel test robot <oliver.sang@...el.com>
>>
>>
>> [   28.582714] BUG: unable to handle page fault for address: fffffffffffffffa
>> [   28.590164] #PF: supervisor read access in kernel mode
>> [   28.590164] #PF: error_code(0x0000) - not-present page
>> [   28.590165] PGD c7e20d067 P4D c7e20d067 PUD c7e20f067 PMD 0
>> [   28.590169] Oops: 0000 [#1] SMP PTI
>> [   28.590171] CPU: 15 PID: 0 Comm: swapper/15 Not tainted 5.10.0-rc2-00373-g0b726f6b318a #1
>> [   28.590172] Hardware name: Supermicro SYS-5018D-FN4T/X10SDV-8C-TLN4F, BIOS 1.1 03/02/2016
>> [   28.590177] RIP: 0010:__udp4_lib_rcv+0x547/0xbe0
>> [   28.590178] Code: 74 0a f6 45 3c 80 74 04 44 8b 4d 28 48 8b 55 58 48 83 e2 fe 74 07 8b 52 7c 85 d2 75 06 8b 95 90 00 00 00 48 8b be f0 04 00 00 <44> 8b 58 0c 8b 48 10 55 41 55 44 89 de 41 51 41 89 d1 44 89 d2 e8
>> [   28.590179] RSP: 0018:ffffc900003b4bb8 EFLAGS: 00010246
>> [   28.590180] RAX: ffffffffffffffee RBX: 0000000000000011 RCX: ffff888c7bc580e2
>> [   28.590181] RDX: 0000000000000002 RSI: ffff88810ddc8000 RDI: ffffffff82d68f00
>> [   28.590182] RBP: ffff888c7bf8f800 R08: 00000000000003b7 R09: 0000000000000000
>> [   28.590182] R10: 0000000000003500 R11: 0000000000000000 R12: ffff888c7bc580e2
>> [   28.590183] R13: ffffffff82e072b0 R14: ffffffff82d68f00 R15: 0000000000000034
>> [   28.590184] FS:  0000000000000000(0000) GS:ffff888c7fdc0000(0000) knlGS:0000000000000000
>> [   28.590185] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [   28.590186] CR2: fffffffffffffffa CR3: 0000000c7e20a006 CR4: 00000000003706e0
>> [   28.590186] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> [   28.590187] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> [   28.590187] Call Trace:
>> [   28.590189]  <IRQ>
>> [   28.590193]  ip_protocol_deliver_rcu+0xc5/0x1c0
>> [   28.590196]  ip_local_deliver_finish+0x4b/0x60
>> [   28.738714]  ip_local_deliver+0x6e/0x140
>> [   28.738717]  ip_sublist_rcv_finish+0x57/0x80
>> [   28.738719]  ip_sublist_rcv+0x199/0x240
>> [   28.750730]  ip_list_rcv+0x13a/0x160
>> [   28.750733]  __netif_receive_skb_list_core+0x2a9/0x2e0
>> [   28.750736]  netif_receive_skb_list_internal+0x1d3/0x320
>> [   28.764743]  gro_normal_list+0x19/0x40
>> [   28.764747]  napi_complete_done+0x68/0x160
>> [   28.773197]  igb_poll+0x63/0x320
>> [   28.773198]  net_rx_action+0x136/0x3a0
>> [   28.773201]  __do_softirq+0xe1/0x2c3
>> [   28.773204]  asm_call_irq_on_stack+0x12/0x20
>> [   28.773205]  </IRQ>
>> [   28.773208]  do_softirq_own_stack+0x37/0x40
>> [   28.773211]  irq_exit_rcu+0xd2/0xe0
>> [   28.773213]  common_interrupt+0x74/0x140
>> [   28.773216]  asm_common_interrupt+0x1e/0x40
>> [   28.773219] RIP: 0010:cpuidle_enter_state+0xd2/0x360
>
> This was expected. This v3 of the patch has already been superseded by
> one that addresses this lookup:

Wait. This page fault happens on IP receive, which is performed after
all GRO processing. At this point, all headers are pulled to skb->head,
and no GRO helpers are needed to access them.
The function that causes that, __udp4_lib_rcv(), uses ip_hdr() a lot,
and it's safe. There should be another questionable point.

>> @@ -534,7 +534,7 @@ static inline struct sock *__udp4_lib_lookup_skb(struct sk_buff *skb,
>>                                                  __be16 sport, __be16 dport,
>>                                                  struct udp_table *udptable)
>>  {
>> -       const struct iphdr *iph = ip_hdr(skb);
>> +       const struct iphdr *iph = skb_gro_network_header(skb);
>
> The merged version was v5 and lacks this change.

Thanks,
Al

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