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

[Will Deacon]

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?

Will