Re: [PATCH 1/4] memory-hotplug: add memory_block_release

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

Hi Kosaki-san,

2012/09/28 10:35, KOSAKI Motohiro wrote:
> On Thu, Sep 27, 2012 at 8:24 PM, Yasuaki Ishimatsu
> <isimatu.yasuaki@jp.fujitsu.com> wrote:
>> Hi Chen,
>>
>>
>> 2012/09/27 19:20, Ni zhan Chen wrote:
>>>
>>> Hi Congyang,
>>>
>>> 2012/9/27 <wency@cn.fujitsu.com>
>>>
>>>> From: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
>>>>
>>>> When calling remove_memory_block(), the function shows following message
>>>> at
>>>> device_release().
>>>>
>>>> Device 'memory528' does not have a release() function, it is broken and
>>>> must
>>>> be fixed.
>>>>
>>>
>>> What's the difference between the patch and original implemetation?
>>
>>
>> The implementation is for removing a memory_block. So the purpose is
>> same as original one. But original code is bad manner. kobject_cleanup()
>> is called by remove_memory_block() at last. But release function for
>> releasing memory_block is not registered. As a result, the kernel message
>> is shown. IMHO, memory_block should be release by the releae function.
>
> but your patch introduced use after free bug, if i understand correctly.
> See unregister_memory() function. After your patch, kobject_put() call
> release_memory_block() and kfree(). and then device_unregister() will
> touch freed memory.

It is not correct. The kobject_put() is prepared against find_memory_block()
in remove_memory_block() since kobject->kref is incremented in it.
So release_memory_block() is called by device_unregister() correctly as follows:

[ 1014.589008] Pid: 126, comm: kworker/0:2 Not tainted 3.6.0-rc3-enable-memory-hotremove-and-root-bridge #3
[ 1014.702437] Call Trace:
[ 1014.731684]  [<ffffffff8144d096>] release_memory_block+0x16/0x30
[ 1014.803581]  [<ffffffff81438587>] device_release+0x27/0xa0
[ 1014.869312]  [<ffffffff8133e962>] kobject_cleanup+0x82/0x1b0
[ 1014.937062]  [<ffffffff8133ea9d>] kobject_release+0xd/0x10
[ 1015.002718]  [<ffffffff8133e7ec>] kobject_put+0x2c/0x60
[ 1015.065271]  [<ffffffff81438107>] put_device+0x17/0x20
[ 1015.126794]  [<ffffffff8143918a>] device_unregister+0x2a/0x60
[ 1015.195578]  [<ffffffff8144d55b>] remove_memory_block+0xbb/0xf0
[ 1015.266434]  [<ffffffff8144d5af>] unregister_memory_section+0x1f/0x30
[ 1015.343532]  [<ffffffff811c0a58>] __remove_section+0x68/0x110
[ 1015.412318]  [<ffffffff811c0be7>] __remove_pages+0xe7/0x120
[ 1015.479021]  [<ffffffff81653d8c>] arch_remove_memory+0x2c/0x80
[ 1015.548845]  [<ffffffff8165497b>] remove_memory+0x6b/0xd0
[ 1015.613474]  [<ffffffff813d946c>] acpi_memory_device_remove_memory+0x48/0x73
[ 1015.697834]  [<ffffffff813d94c2>] acpi_memory_device_remove+0x2b/0x44
[ 1015.774922]  [<ffffffff813a61e4>] acpi_device_remove+0x90/0xb2
[ 1015.844796]  [<ffffffff8143c2fc>] __device_release_driver+0x7c/0xf0
[ 1015.919814]  [<ffffffff8143c47f>] device_release_driver+0x2f/0x50
[ 1015.992753]  [<ffffffff813a70dc>] acpi_bus_remove+0x32/0x6d
[ 1016.059462]  [<ffffffff813a71a8>] acpi_bus_trim+0x91/0x102
[ 1016.125128]  [<ffffffff813a72a1>] acpi_bus_hot_remove_device+0x88/0x16b
[ 1016.204295]  [<ffffffff813a2e57>] acpi_os_execute_deferred+0x27/0x34
[ 1016.280350]  [<ffffffff81090599>] process_one_work+0x219/0x680
[ 1016.350173]  [<ffffffff81090538>] ? process_one_work+0x1b8/0x680
[ 1016.422072]  [<ffffffff813a2e30>] ? acpi_os_wait_events_complete+0x23/0x23
[ 1016.504357]  [<ffffffff810923ce>] worker_thread+0x12e/0x320
[ 1016.571064]  [<ffffffff810922a0>] ? manage_workers+0x110/0x110
[ 1016.640886]  [<ffffffff810983a6>] kthread+0xc6/0xd0
[ 1016.699290]  [<ffffffff8167b144>] kernel_thread_helper+0x4/0x10
[ 1016.770149]  [<ffffffff81670bb0>] ? retint_restore_args+0x13/0x13
[ 1016.843165]  [<ffffffff810982e0>] ? __init_kthread_worker+0x70/0x70
[ 1016.918200]  [<ffffffff8167b140>] ? gs_change+0x13/0x13

Thanks,
Yasuaki Ishimatsu

>
> static void
> unregister_memory(struct memory_block *memory)
> {
> 	BUG_ON(memory->dev.bus != &memory_subsys);
>
> 	/* drop the ref. we got in remove_memory_block() */
> 	kobject_put(&memory->dev.kobj);
> 	device_unregister(&memory->dev);
> }
>


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