linux-kernel - Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte

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Date:   Mon, 30 Jan 2023 13:46:10 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mark Rutland <mark.rutland@....com>, Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        robin.murphy@....com
Subject: Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at()



On 1/27/23 20:46, Mark Rutland wrote:
> On Thu, Jan 26, 2023 at 01:33:22PM +0000, Will Deacon wrote:
>> On Tue, Jan 24, 2023 at 11:11:49AM +0530, Anshuman Khandual wrote:
>>> On 1/9/23 10:58, Anshuman Khandual wrote:
>>>> Changing pfn on a user page table mapped entry, without first going through
>>>> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
>>>> to intercept such changes, via an updated pgattr_change_is_safe(). This new
>>>> check happens via __check_racy_pte_update(), which has now been renamed as
>>>> __check_safe_pte_update().
>>>>
>>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>>> Cc: Will Deacon <will@...nel.org>
>>>> Cc: Mark Rutland <mark.rutland@....com>
>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>> Cc: linux-arm-kernel@...ts.infradead.org
>>>> Cc: linux-kernel@...r.kernel.org
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>>> ---
>>>> This applies on v6.2-rc3. This patch had some test time on an internal CI
>>>> system without any issues being reported.
>>>
>>> Gentle ping, any updates on this patch ? Still any concerns ?
>>
>> I don't think we really got to the bottom of Mark's concerns with
>> unreachable ptes on the stack, did we? I also have vague recollections
>> of somebody (Robin?) running into issues with the vmap code not honouring
>> BBM.
>>
>> So I think we should confirm/fix the vmap issue before we enable this check
>> and also try to get some testing coverage to address Mark's worries. I think
>> he has a syzkaller instance set up, so that sound like a good place to
>> start.
> 
> I've thrown my Syzkaller instance at this patch; if it doesn't find anything by
> Monday I reckon we should pick this up.
> 
> That said, I had some minor nits on the patch; I'm not sure if you'd be happy
> to apply the suggested changes when applying or if you'd prefer that Anshuman
> applies those locally and sense a v3.

I could send out a V3, running some stress-ng based memory tests with the suggested
changes applied on the patch.