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Date: Thu, 8 May 2014 19:55:34 +0800
From: Wang Weidong <wangweidong1@...wei.com>
To: Neil Horman <nhorman@...driver.com>
CC: Vlad Yasevich <vyasevich@...il.com>,
David Miller <davem@...emloft.net>,
Daniel Borkmann <dborkman@...hat.com>,
<linux-sctp@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] sctp: fix kfree static array pointer in sctp_sysctl_net_unregister
On 2014/5/8 19:47, Neil Horman wrote:
> On Thu, May 08, 2014 at 07:26:31PM +0800, Wang Weidong wrote:
>> On 2014/5/8 19:10, Neil Horman wrote:
>>> On Thu, May 08, 2014 at 03:39:06PM +0800, Wang Weidong wrote:
>>>> As commit efb842c45e("sctp: optimize the sctp_sysctl_net_register"),
>>>> we don't kmemdup a sysctl_table for init_net, so the
>>>> init_net->sctp.sysctl_header->ctl_table_arg points to sctp_net_table
>>>> which is a static array pointer. So when doing sctp_sysctl_net_unregister,
>>>> it will free sctp_net_table, then we will get a NULL pointer dereference
>>>> like that:
>>>>
>>>> [ 262.948220] BUG: unable to handle kernel NULL pointer dereference at 000000000000006c
>>>> [ 262.948232] IP: [<ffffffff81144b70>] kfree+0x80/0x420
>>>> [ 262.948260] PGD db80a067 PUD dae12067 PMD 0
>>>> [ 262.948268] Oops: 0000 [#1] SMP
>>>> [ 262.948273] Modules linked in: sctp(-) crc32c_generic libcrc32c
>>>> ...
>>>> [ 262.948338] task: ffff8800db830190 ti: ffff8800dad00000 task.ti: ffff8800dad00000
>>>> [ 262.948344] RIP: 0010:[<ffffffff81144b70>] [<ffffffff81144b70>] kfree+0x80/0x420
>>>> [ 262.948353] RSP: 0018:ffff8800dad01d88 EFLAGS: 00010046
>>>> [ 262.948358] RAX: 0100000000000000 RBX: ffffffffa0227940 RCX: ffffea0000707888
>>>> [ 262.948363] RDX: ffffea0000707888 RSI: 0000000000000001 RDI: ffffffffa0227940
>>>> [ 262.948369] RBP: ffff8800dad01de8 R08: 0000000000000000 R09: ffff8800d9e983a9
>>>> [ 262.948374] R10: 0000000000000000 R11: 0000000000000000 R12: ffffffffa0227940
>>>> [ 262.948380] R13: ffffffff8187cfc0 R14: 0000000000000000 R15: ffffffff8187da10
>>>> [ 262.948386] FS: 00007fa2a2658700(0000) GS:ffff880112800000(0000) knlGS:0000000000000000
>>>> [ 262.948394] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>>>> [ 262.948400] CR2: 000000000000006c CR3: 00000000cddc0000 CR4: 00000000000006e0
>>>> [ 262.948410] Stack:
>>>> [ 262.948413] ffff8800dad01da8 0000000000000286 0000000020227940 ffffffffa0227940
>>>> [ 262.948422] ffff8800dad01dd8 ffffffff811b7fa1 ffffffffa0227940 ffffffffa0227940
>>>> [ 262.948431] ffffffff8187d960 ffffffff8187cfc0 ffffffff8187d960 ffffffff8187da10
>>>> [ 262.948440] Call Trace:
>>>> [ 262.948457] [<ffffffff811b7fa1>] ? unregister_sysctl_table+0x51/0xa0
>>>> [ 262.948476] [<ffffffffa020d1a1>] sctp_sysctl_net_unregister+0x21/0x30 [sctp]
>>>> [ 262.948490] [<ffffffffa020ef6d>] sctp_net_exit+0x12d/0x150 [sctp]
>>>> [ 262.948512] [<ffffffff81394f49>] ops_exit_list+0x39/0x60
>>>> [ 262.948522] [<ffffffff813951ed>] unregister_pernet_operations+0x3d/0x70
>>>> [ 262.948530] [<ffffffff81395292>] unregister_pernet_subsys+0x22/0x40
>>>> [ 262.948544] [<ffffffffa020efcc>] sctp_exit+0x3c/0x12d [sctp]
>>>> [ 262.948562] [<ffffffff810c5e04>] SyS_delete_module+0x194/0x210
>>>> [ 262.948577] [<ffffffff81240fde>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>>>> [ 262.948587] [<ffffffff815217a2>] system_call_fastpath+0x16/0x1b
>>>>
>>>> So add a check net_namespace init_net before kfree the sysctl_table.
>>>>
>>>> Signed-off-by: Wang Weidong <wangweidong1@...wei.com>
>>>> ---
>>>> net/sctp/sysctl.c | 3 ++-
>>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
>>>> index c82fdc1..844d2b0 100644
>>>> --- a/net/sctp/sysctl.c
>>>> +++ b/net/sctp/sysctl.c
>>>> @@ -459,7 +459,8 @@ void sctp_sysctl_net_unregister(struct net *net)
>>>>
>>>> table = net->sctp.sysctl_header->ctl_table_arg;
>>>> unregister_net_sysctl_table(net->sctp.sysctl_header);
>>>> - kfree(table);
>>>> + if (!net_eq(net, &init_net))
>>>> + kfree(table);
>>>> }
>>>>
>>>> static struct ctl_table_header *sctp_sysctl_header;
>>>> --
>>>> 1.7.12
>>>>
>>>>
>>>>
>>> The size of of the sysctl table is 1.6k at the moment. Is it really worth having
>>> to special case this in two location just to save that space? It almost seems
>>> better just to revert the origonal commit.
>>>
>> I am not sure for that. In kernel, some do with this while others don't.
>> Add the special case, avoid to do kmemdup and reinitialization as well.
>>
> What? No, the size of a ctl_table entry is about 64 bytes, and there are 25
> entries in the table, thats 1.6k per kmemdup. Your origonal patch avoids
> kmemdup for init_net, saving us 1.6k, but creating the need to special case it
> in at least 3 places now. I'm argunig that its maybe better to just accept the
> additional 1.6k, and not have to worry about the special casing.
> Neil
>
OK, Got it.
About "In kernel, some do with this while others don't.", It means that
in ipv4_sysctl_init_net/xfrm4_net_init and so on, take the init_net case
into account.
I will send the revert patch.
Thanks
Wang
>> Regards
>> Wang
>>
>>> Neil
>>>
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>>> .
>>>
>>
>>
>>
>
> .
>
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