linux-kernel - Re: [PATCH v2 10/10] arm64: mm: Unmap kernel data/bss entirely from the linear map

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Message-ID: <2edde60e-b9fa-4458-b1be-ce786e78ff14@arm.com>
Date: Tue, 27 Jan 2026 11:02:50 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, will@...nel.org,
 catalin.marinas@....com, mark.rutland@....com,
 Anshuman Khandual <anshuman.khandual@....com>,
 Liz Prucka <lizprucka@...gle.com>, Seth Jenkins <sethjenkins@...gle.com>,
 Kees Cook <kees@...nel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 10/10] arm64: mm: Unmap kernel data/bss entirely from
 the linear map

On 27/01/2026 10:54, Ard Biesheuvel wrote:
> On Tue, 27 Jan 2026 at 11:50, Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> On 26/01/2026 09:26, Ard Biesheuvel wrote:
>>> From: Ard Biesheuvel <ardb@...nel.org>
>>>
>>> The linear aliases of the kernel text and rodata are mapped read-only in
>>> the linear map as well. Given that the contents of these regions are
>>> mostly identical to the version in the loadable image, mapping them
>>> read-only and leaving their contents visible is a reasonable hardening
>>> measure.
>>
>> what about ro_after_init? Could that contain secrets that we don't want to leak?
>> What is the advantage of leaving text/rodata ro in the linear map vs just
>> umapping the whole lot?
>>
> 
> Unmapping it would be preferred, but there are some pieces (such as
> the zero page and swapper_pg_dir) that are expected to be visible via
> the linear map.
> 
>>>
>>> Data and bss, however, are now also mapped read-only but the contents of
>>> these regions are more likely to contain data that we'd rather not leak.
>>> So let's unmap these entirely in the linear map when the kernel is
>>> running normally.
>>>
>>> When going into hibernation or waking up from it, these regions need to
>>> be mapped, so map the region initially, and toggle the valid bit so
>>> map/unmap the region as needed.
>>>
>>> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>>> ---
>>>  arch/arm64/mm/mmu.c | 40 ++++++++++++++++++--
>>>  1 file changed, 37 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index fdbbb018adc5..06b2d11b4561 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -24,6 +24,7 @@
>>>  #include <linux/mm.h>
>>>  #include <linux/vmalloc.h>
>>>  #include <linux/set_memory.h>
>>> +#include <linux/suspend.h>
>>>  #include <linux/kfence.h>
>>>  #include <linux/pkeys.h>
>>>  #include <linux/mm_inline.h>
>>> @@ -1027,6 +1028,31 @@ static void __init __map_memblock(phys_addr_t start, phys_addr_t end,
>>>                                end - start, prot, early_pgtable_alloc, flags);
>>>  }
>>>
>>> +static void remap_linear_data_alias(bool unmap)
>>> +{
>>> +     set_memory_valid((unsigned long)lm_alias(__init_end),
>>> +                      (unsigned long)(__pgdir_start - __init_end) / PAGE_SIZE,
>>> +                      !unmap);
>>> +}
>>> +
>>> +static int arm64_hibernate_pm_notify(struct notifier_block *nb,
>>> +                                  unsigned long mode, void *unused)
>>> +{
>>> +     switch (mode) {
>>> +     default:
>>> +             break;
>>> +     case PM_POST_HIBERNATION:
>>> +     case PM_POST_RESTORE:
>>> +             remap_linear_data_alias(true);
>>> +             break;
>>> +     case PM_HIBERNATION_PREPARE:
>>> +     case PM_RESTORE_PREPARE:
>>> +             remap_linear_data_alias(false);
>>> +             break;
>>> +     }
>>> +     return 0;
>>> +}
>>> +
>>>  void __init mark_linear_text_alias_ro(void)
>>>  {
>>>       /*
>>> @@ -1035,6 +1061,16 @@ void __init mark_linear_text_alias_ro(void)
>>>       update_mapping_prot(__pa_symbol(_text), (unsigned long)lm_alias(_text),
>>>                           (unsigned long)__init_begin - (unsigned long)_text,
>>>                           PAGE_KERNEL_RO);
>>> +
>>> +     remap_linear_data_alias(true);
>>> +
>>> +     if (IS_ENABLED(CONFIG_HIBERNATION)) {
>>> +             static struct notifier_block nb = {
>>> +                     .notifier_call = arm64_hibernate_pm_notify
>>> +             };
>>> +
>>> +             register_pm_notifier(&nb);
>>> +     }
>>>  }
>>>
>>>  #ifdef CONFIG_KFENCE
>>> @@ -1163,7 +1199,7 @@ static void __init map_mem(void)
>>>       __map_memblock(kernel_start, init_begin, PAGE_KERNEL,
>>>                      flags | NO_CONT_MAPPINGS);
>>>       __map_memblock(init_end, kernel_end, PAGE_KERNEL,
>>> -                    flags | NO_CONT_MAPPINGS);
>>> +                    flags | NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
>>>
>>>       /* map all the memory banks */
>>>       for_each_mem_range(i, &start, &end) {
>>> @@ -1176,8 +1212,6 @@ static void __init map_mem(void)
>>>                              flags);
>>>       }
>>>
>>> -     __map_memblock(init_end, kernel_end, PAGE_KERNEL_RO,
>>> -                    flags | NO_CONT_MAPPINGS);
>>
>> In the previous patch, perhaps this should be moved to
>> mark_linear_text_alias_ro() and combined with the update_mapping_prot() there?
>>
> 
> Why? Isn't it better to just remap it r/o from the outset if it never
> needs to be mapped r/w to begin with?
> 
> The same applies to the rodata region but -again- it is the root page
> tables that are manipulated via the writable linear alias during boot.
> 
> I'll try to address all of that in the next rev - thanks.

Cool, I'll look out for it.