Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at()
From: Robin Murphy
Date: Fri Jan 27 2023 - 07:43:35 EST
On 2023-01-26 13:33, Will Deacon wrote:
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.
Doesn't ring a bell, so either it wasn't me, or it was many years ago
and about 5 levels deep into trying to fix something else :/
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.
I think we're also missing a subtlety here in that this restriction
doesn't *always* apply. For instance if someone wants to move a page by
making the mapping read-only, copying the contents to a new page, then
pointing the RO mapping at that new page, that should technically not
require BBM.
Thanks,
Robin.