Subject: Re: [PATCH V7 2/3] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

From: Anshuman Khandual <[email-masked]>
Date: Wed, 18 Sep 2019 14:42:47 +0530

On 09/15/2019 08:05 AM, Balbir Singh wrote:
> 
> 
> On 3/9/19 7:45 pm, Anshuman Khandual wrote:
>> The arm64 page table dump code can race with concurrent modification of the
>> kernel page tables. When a leaf entries are modified concurrently, the dump
>> code may log stale or inconsistent information for a VA range, but this is
>> otherwise not harmful.
>>
>> When intermediate levels of table are freed, the dump code will continue to
>> use memory which has been freed and potentially reallocated for another
>> purpose. In such cases, the dump code may dereference bogus addresses,
>> leading to a number of potential problems.
>>
>> Intermediate levels of table may by freed during memory hot-remove,
>> which will be enabled by a subsequent patch. To avoid racing with
>> this, take the memory hotplug lock when walking the kernel page table.
>>
>> Acked-by: David Hildenbrand <[email-masked]>
>> Acked-by: Mark Rutland <[email-masked]>
>> Signed-off-by: Anshuman Khandual <[email-masked]>
>> ---
>>  arch/arm64/mm/ptdump_debugfs.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm64/mm/ptdump_debugfs.c b/arch/arm64/mm/ptdump_debugfs.c
>> index 064163f25592..b5eebc8c4924 100644
>> --- a/arch/arm64/mm/ptdump_debugfs.c
>> +++ b/arch/arm64/mm/ptdump_debugfs.c
>> @@ -1,5 +1,6 @@
>>  // SPDX-License-Identifier: GPL-2.0
>>  #include <linux/debugfs.h>
>> +#include <linux/memory_hotplug.h>
>>  #include <linux/seq_file.h>
>>  
>>  #include <asm/ptdump.h>
>> @@ -7,7 +8,10 @@
>>  static int ptdump_show(struct seq_file *m, void *v)
>>  {
>>  	struct ptdump_info *info = m->private;
>> +
>> +	get_online_mems();
>>  	ptdump_walk_pgd(m, info);
>> +	put_online_mems();
> 
> Looks sane, BTW, checking other arches they might have the same race.

The problem can be present on other architectures which can dump kernel page
table during memory hot-remove operation where it actually frees up page table
pages. If there is no freeing involved the race condition here could cause
inconsistent or garbage information capture for a given VA range. Same is true
even for concurrent vmalloc() operations as well. But removal of page tables
pages can make it worse. Freeing page table pages during hot-remove is a platform
decision, so would be adding these locks while walking kernel page table during
ptdump.

> Is there anything special about the arch?

AFAICS, no.

> 
> Acked-by: Balbir Singh <[email-masked]>
> 
> 

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