Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Anshuman Khandual]

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@xxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Mark Rutland <mark.rutland@xxxxxxx> 
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
>> 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.