From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Balbir Singh <bsingharora@gmail.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org,
catalin.marinas@arm.com, will@kernel.org
Cc: mark.rutland@arm.com, mhocko@suse.com, david@redhat.com,
ira.weiny@intel.com, steve.capper@arm.com,
mgorman@techsingularity.net, steven.price@arm.com,
broonie@kernel.org, cai@lca.pw, ard.biesheuvel@arm.com,
cpandya@codeaurora.org, arunks@codeaurora.org,
dan.j.williams@intel.com, Robin.Murphy@arm.com,
logang@deltatee.com, valentin.schneider@arm.com,
suzuki.poulose@arm.com, osalvador@suse.de
Subject: Re: [PATCH V7 2/3] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump
Date: Wed, 18 Sep 2019 14:42:47 +0530 [thread overview]
Message-ID: <8e47831f-c28e-a174-24b3-b3bbf1f365ec@arm.com> (raw)
In-Reply-To: <66922798-9de7-a230-8548-1f205e79ea50@gmail.com>
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 <david@redhat.com>
>> Acked-by: Mark Rutland <mark.rutland@arm.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> 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 <bsingharora@gmail.com>
>
>
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next prev parent reply other threads:[~2019-09-18 9:12 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-03 9:45 [PATCH V7 0/3] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-09-03 9:45 ` [PATCH V7 1/3] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory() Anshuman Khandual
2019-09-04 8:16 ` David Hildenbrand
2019-09-05 4:27 ` Anshuman Khandual
2019-09-16 1:44 ` Balbir Singh
2019-09-18 9:28 ` Anshuman Khandual
2019-09-03 9:45 ` [PATCH V7 2/3] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump Anshuman Khandual
2019-09-15 2:35 ` Balbir Singh
2019-09-18 9:12 ` Anshuman Khandual [this message]
2019-09-03 9:45 ` [PATCH V7 3/3] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-09-10 16:17 ` Catalin Marinas
2019-09-11 10:31 ` David Hildenbrand
2019-09-12 4:28 ` Anshuman Khandual
2019-09-12 8:37 ` Anshuman Khandual
2019-09-12 20:15 ` Catalin Marinas
2019-09-13 5:58 ` Anshuman Khandual
2019-09-13 10:09 ` Catalin Marinas
2019-09-17 4:36 ` Anshuman Khandual
2019-09-17 15:08 ` Catalin Marinas
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