Messages in this thread | | | Date | Fri, 27 Jan 2023 12:43:17 +0000 | Subject | Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at() | From | Robin Murphy <> |
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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@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 v6.2-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.
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