Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at()

[Will Deacon]

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@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 v6.2-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.

Will