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Message-ID: <879e561c-e834-196c-b9c5-6e44ac2c0296@arm.com>
Date:   Tue, 22 Nov 2022 13:43:17 +0530
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Subject: Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()
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To:     Will Deacon <will@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com,
        Mark Rutland <mark.rutland@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
References: <20221116031001.292236-1-anshuman.khandual@arm.com>
 <20221118141317.GF4046@willie-the-truck>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
In-Reply-To: <20221118141317.GF4046@willie-the-truck>
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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@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com> 
>> Cc: Andrew Morton <akpm@linux-foundation.org> 
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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.