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

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Ard Biesheuvel <ardb@kernel.org>, Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
	Steve Capper <steve.capper@arm.com>,
	Mark Brown <broonie@kernel.org>, Marc Zyngier <maz@kernel.org>,
	gshan@redhat.com, Robin Murphy <robin.murphy@arm.com>,
	Steven Price <steven.price@arm.com>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] arm64: mm: account for hotplug memory when randomizing the linear region
Date: Fri, 13 Nov 2020 08:46:27 +0530	[thread overview]
Message-ID: <2f0d9bc5-6288-7388-ff10-97198dabac6f@arm.com> (raw)
In-Reply-To: <20201112092558.GC29613@gaia>

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/

> 
>> 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.