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

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Message-ID: <879e561c-e834-196c-b9c5-6e44ac2c0296@arm.com>
Date:   Tue, 22 Nov 2022 13:43:17 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()



On 11/18/22 19:43, Will Deacon wrote:
> On Wed, Nov 16, 2022 at 08:40:01AM +0530, 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 v6.1-rc4
>>
>>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>>  arch/arm64/mm/mmu.c              | 8 +++++++-
>>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> I remember Mark saying that BBM is sometimes violated by the core code in
> cases where the pte isn't actually part of a live pgtable (e.g. if it's on
> the stack or part of a newly allocated table). Won't that cause false
> positives here?

Could you please elaborate ? If the pte is not on a live page table, then
pte_valid() will return negative on such entries. So any update there will
be safe. I am wondering, how this change will cause false positives which
would not have been possible earlier.