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Wed, 29 Jan 2025 15:31:46 -0800 (PST) Received: from [10.67.48.245] ([192.19.223.252]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-72fe69cdf76sm33681b3a.129.2025.01.29.15.31.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Jan 2025 15:31:45 -0800 (PST) Message-ID: <8994e7c5-812c-4605-9bdf-18a5b402196a@broadcom.com> Date: Wed, 29 Jan 2025 15:31:40 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] arm64: mm: account for hotplug memory when randomizing the linear region To: Ard Biesheuvel Cc: Greg KH , stable@vger.kernel.org, Anshuman Khandual , Will Deacon , Steven Price , Robin Murphy , Catalin Marinas , Baruch Siach , Petr Tesarik , Mark Rutland , Joey Gouly , "Mike Rapoport (IBM)" , Yang Shi , "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" , open list References: <20250109165419.1623683-1-florian.fainelli@broadcom.com> <20250109165419.1623683-2-florian.fainelli@broadcom.com> <62786457-d4a1-4861-8bec-7e478626f4db@broadcom.com> <2025011247-enable-freezing-ffa2@gregkh> <27bbea11-61fa-4f41-8b39-8508f2d2e385@broadcom.com> <2025012002-tactics-murky-aaab@gregkh> <41550c7f-1313-41b4-aa2e-cb4809ad68c2@broadcom.com> <2025012938-abreast-explain-f5f7@gregkh> <1fc6d5c8-80ec-4d6b-bc14-c584d89c15b4@broadcom.com> Content-Language: en-US From: Florian Fainelli Autocrypt: addr=florian.fainelli@broadcom.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/29/25 14:15, Ard Biesheuvel wrote: > On Wed, 29 Jan 2025 at 18:45, Florian Fainelli > wrote: >> >> On 1/29/25 01:17, Greg KH wrote: >>> On Mon, Jan 20, 2025 at 08:33:12AM -0800, Florian Fainelli wrote: >>>> >>>> >>>> On 1/20/2025 5:59 AM, Greg KH wrote: >>>>> On Mon, Jan 13, 2025 at 07:44:50AM -0800, Florian Fainelli wrote: >>>>>> >>>>>> >>>>>> On 1/12/2025 3:54 AM, Greg KH wrote: >>>>>>> On Thu, Jan 09, 2025 at 09:01:13AM -0800, Florian Fainelli wrote: >>>>>>>> On 1/9/25 08:54, Florian Fainelli wrote: >>>>>>>>> From: Ard Biesheuvel >>>>>>>>> >>>>>>>>> commit 97d6786e0669daa5c2f2d07a057f574e849dfd3e upstream >>>>>>>>> >>>>>>>>> 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. >>>>>>>>> >>>>>>>>> So let's limit this randomization of the linear region to ensure >>>>>>>>> that this can no longer happen, by using the CPU's addressable PA >>>>>>>>> range instead. As it is guaranteed that no hotpluggable memory will >>>>>>>>> appear that falls outside of that range, we can safely put this PA >>>>>>>>> range sized window anywhere in the linear region. >>>>>>>>> >>>>>>>>> Signed-off-by: Ard Biesheuvel >>>>>>>>> Cc: Anshuman Khandual >>>>>>>>> Cc: Will Deacon >>>>>>>>> Cc: Steven Price >>>>>>>>> Cc: Robin Murphy >>>>>>>>> Link: https://lore.kernel.org/r/20201014081857.3288-1-ardb@kernel.org >>>>>>>>> Signed-off-by: Catalin Marinas >>>>>>>>> Signed-off-by: Florian Fainelli >>>>>>>> >>>>>>>> Forgot to update the patch subject, but this one is for 5.10. >>>>>>> >>>>>>> You also forgot to tell us _why_ this is needed :( >>>>>> >>>>>> This is explained in the second part of the first paragraph: >>>>>> >>>>>> 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. >>>>>> >>>>>> We use both memory hotplug and KASLR on our systems and that's how we >>>>>> eventually found out about the bug. >>>>> >>>>> And you still have 5.10.y ARM64 systems that need this? Why not move to >>>>> a newer kernel version already? >>>> >>>> We still have ARM64 systems running 5.4 that need this, and the same bug >>>> applies to 5.10 that we used to support but dropped in favor of 5.15/6.1. >>>> Those are the kernel versions used by Android, and Android TV in particular, >>>> so it's kind of the way it goes for us. >>>> >>>>> >>>>> Anyway, I need an ack from the ARM64 maintainers that this is ok to >>>>> apply here before I can take it. >>>> >>>> Just out of curiosity, the change is pretty innocuous and simple to review, >>>> why the extra scrutiny needed here? >>> >>> Why shouldn't the maintainers review a proposed backport patch for core >>> kernel code that affects everyone who uses that arch? >> >> They should, but they are not, we can keep sending messages like those >> in the hope that someone does, but clearly that is not working at the >> moment. >> >> This patch cherry picked cleanly into 5.4 and 5.10 maybe they just trust >> whoever submit stable bugfixes to have done their due diligence, too, I >> don't know how to get that moving now but it fixes a real problem we >> observed. >> > > FWIW, I understand why this might be useful when running under a > non-KVM hypervisor that relies on memory hotplug to perform resource > balancing between VMs. But the upshot of this change is that existing > systems that do not rely on memory hotplug at all will suddenly lose > any randomization of the linear map if its CPU happens to be able to > address more than ~40 bits of physical memory. So I'm not convinced > this is a change we should make for these older kernels. Are there other patches that we could backport in order not to lose the randomization in the linear range? -- Florian