linux-kernel - Re: [PATCH V14 1/2] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

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Message-ID: <91e4cbd0-0f1d-30c6-2796-d2fb91ba6720@arm.com>
Date:   Wed, 4 Mar 2020 10:12:46 +0000
From:   Steven Price <steven.price@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        "david@...hat.com" <david@...hat.com>, "cai@....pw" <cai@....pw>,
        "logang@...tatee.com" <logang@...tatee.com>,
        "arunks@...eaurora.org" <arunks@...eaurora.org>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "osalvador@...e.de" <osalvador@...e.de>,
        Ard Biesheuvel <Ard.Biesheuvel@....com>,
        Steve Capper <Steve.Capper@....com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        Valentin Schneider <Valentin.Schneider@....com>,
        Robin Murphy <Robin.Murphy@....com>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>
Subject: Re: [PATCH V14 1/2] arm64/mm: Hold memory hotplug lock while walking
 for kernel page table dump

On 04/03/2020 04:28, 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@...hat.com>
> Acked-by: Mark Rutland <mark.rutland@....com>
> Acked-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>

Reviewed-by: Steven Price <steven.price@....com>

Thanks,

Steve

> ---
>  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 1f2eae3e988b..d29d722ec3ec 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(m, info);
> +	put_online_mems();
>  	return 0;
>  }
>  DEFINE_SHOW_ATTRIBUTE(ptdump);
>