Re: [PATCH] arm64: mm: account for hotplug memory when randomizing the linear region

From: Ard Biesheuvel <ardb@kernel.org>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	gshan@redhat.com, Steve Capper <steve.capper@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	David Hildenbrand <david@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Steven Price <steven.price@arm.com>,
	Mark Brown <broonie@kernel.org>, Marc Zyngier <maz@kernel.org>,
	Will Deacon <will@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH] arm64: mm: account for hotplug memory when randomizing the linear region
Date: Fri, 13 Nov 2020 07:14:54 +0100	[thread overview]
Message-ID: <CAMj1kXGa-WUr8LzDHGEfgG18ctJJp1v8b4UFckbcwtzpoEv+Tw@mail.gmail.com> (raw)
In-Reply-To: <2f0d9bc5-6288-7388-ff10-97198dabac6f@arm.com>

On Fri, 13 Nov 2020 at 04:16, Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
>
>
> On 11/12/20 2:55 PM, Catalin Marinas wrote:
> > Hi Anshuman,
> >
> > On Wed, Nov 11, 2020 at 09:18:56AM +0530, Anshuman Khandual wrote:
> >> On 11/11/20 12:44 AM, Catalin Marinas wrote:
> >>> On Wed, 14 Oct 2020 10:18:57 +0200, Ard Biesheuvel wrote:
> >>>> As a hardening measure, we currently randomize the placement of
> >>>> physical memory inside the linear region when KASLR is in effect.
> >>>> Since the random offset at which to place the available physical
> >>>> memory inside the linear region is chosen early at boot, it is
> >>>> based on the memblock description of memory, which does not cover
> >>>> hotplug memory. The consequence of this is that the randomization
> >>>> offset may be chosen such that any hotplugged memory located above
> >>>> memblock_end_of_DRAM() that appears later is pushed off the end of
> >>>> the linear region, where it cannot be accessed.
> >>>>
> >>>> [...]
> >>>
> >>> Applied to arm64 (for-next/mem-hotplug), thanks!
> >>>
> >>> [1/1] arm64: mm: account for hotplug memory when randomizing the linear region
> >>>       https://git.kernel.org/arm64/c/97d6786e0669
> >>
> >> Got delayed and never made here in time, sorry about that. Nonetheless,
> >> I have got something working with respect to the generic mechanism that
> >> David Hildenbrand had asked for earlier.
> >>
> >> https://patchwork.kernel.org/project/linux-arm-kernel/patch/1600332402-30123-1-git-send-email-anshuman.khandual@arm.com/
> >
> > There was a lot of discussion around this patch but I haven't seen any
> > new version posted.
>
> Just posted before some time.
>
> https://lore.kernel.org/linux-arm-kernel/1605236574-14636-1-git-send-email-anshuman.khandual@arm.com/
>

You failed to cc me on that patch.

The logic looks correct but please fix up the comment block:
- PAGE_END is no longer defined in terms of vabits_actual
- bits [51..48] are not ignored by the MMU

Actually, I think the entire second paragraph of that comment block
can be dropped.

Please also fix up the coding style:
- put && at the end of the first line
- drop the redundant parens
- fix the indentation

> >
> >> I am wondering if we could instead consider merging the above patch with
> >> a small change that Ard had pointed out earlier [1], I will send out a
> >> revision if required.
> >
> > If your patch fixes the randomisation issue that Ard addressed, I'm
> > happy to replace that with your patch. But please post a new version and
> > get some acks in place from the parties involved in the discussion.
>
> The above patch is not a replacement for Ard's randomization patch here but
> rather complements it. Hence both these patches should be considered, which
> will make memory hotplug better on the platform.

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