Re: [PATCH v7 00/15] arm64: Unmap linear alias of kernel data/bss

On Wed, 3 Jun 2026, at 13:22, Will Deacon wrote:
> On Wed, Jun 03, 2026 at 10:57:49AM +0200, Ard Biesheuvel wrote:
>> (cc Marc)
>> 
>> On Tue, 2 Jun 2026, at 22:34, Will Deacon wrote:
>> > On Fri, 29 May 2026 17:01:51 +0200, Ard Biesheuvel wrote:
>> >> One of the reasons the lack of randomization of the linear map on arm64
>> >> is considered problematic is the fact that bootloaders adhering to the
>> >> original arm64 boot protocol (i.e., a substantial fraction of all
>> >> Android phones) may place the kernel at the base of DRAM, and therefore
>> >> at the base of the non-randomized linear map. This puts a writable alias
>> >> of the kernel's data and bss regions at a predictable location, removing
>> >> the need for an attacker to guess where KASLR mapped the kernel.
>> >> 
>> >> [...]
>> >
>> > It would've been nice to hear from the ppc folks on patch 11, but I've
>> > picked it up on the assumption that they'll love the negative diff stat.
>> > Worst case, we can drop/revert stuff if they have late objections.
>> >
>> 
>> Thanks.
>> 
>> There is a de facto ack from Michael Ellerman in the Link:, which is why
>> I included it.
>> 
>> Note that Sashiko found an issue with KVM+MTE, where a read-only mapping
>> of the zero page in the linear map may result in issues:
>> 
>> """
>> Does moving the zero page to .rodata (or unmapping/read-only mapping its
>> linear alias) expose a guest-to-host denial of service with KVM and MTE?
>> When an MTE-enabled KVM guest reads an unmapped memory address, KVM handles
>> the stage-2 fault by mapping the host's shared zero page. KVM will then
>> call sanitise_mte_tags() in arch/arm64/kvm/mmu.c.
>> Since the PG_mte_tagged flag is never set on the zero page, KVM's
>> try_page_mte_tagging() succeeds, and it calls mte_clear_page_tags().
>> This executes the STGM instruction using the zero page's linear map alias.
>> If this alias is read-only or unmapped, won't the STGM instruction trigger
>> a synchronous permission fault or translation fault in EL1, causing a host
>> kernel panic?
>> """
>> 
>> Marc seems to think it is legit, so I came up with the following (I'll send
>> it out separately with another pair of tweaks):
>
> Thanks, it also looks like we're getting some early WARN_ON()s firing in
> CI from split_kernel_leaf_mapping() after applying your changes:
>
> https://s3.amazonaws.com/arr-cki-prod-trusted-artifacts/trusted-artifacts/2571596185/test_aarch64/14662134813/artifacts/jobwatch/logs/recipes/21399931/tasks/219104268/results/1007729692/logs/journalctl.log
>

OK I'll investigate