Re: [PATCH v6 15/15] arm64: mm: Unmap kernel data/bss entirely from the linear map

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 26/05/2026 19:59, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.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.
>
> 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. (While the hibernation snapshot logic
> seems able to map inaccessible pages as needed, it currently disregards
> non-present pages entirely.)

I'm not sure I understand this, is there something wrong with the
kernel_page_present() check in safe_copy_page()?

> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/mm/mmu.c | 39 +++++++++++++++++---
>  1 file changed, 34 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index e7ca53d20b87..cb00e42abbe1 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>
> @@ -1056,6 +1057,29 @@ 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)(__bss_stop - __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:
> +		remap_linear_data_alias(true);
> +		break;
> +	case PM_HIBERNATION_PREPARE:
> +		remap_linear_data_alias(false);
> +		break;
> +	}
> +	return 0;
> +}
> +
>  void __init mark_linear_text_alias_ro(void)
>  {
>  	/*
> @@ -1064,6 +1088,16 @@ 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);

As suggested on v4, something like mark_linear_data_alias_valid(false)
would be clearer.

Also, is there anything stopping us from doing that directly in map_mem()?

- Kevin

> +
> +	if (IS_ENABLED(CONFIG_HIBERNATION)) {
> +		static struct notifier_block nb = {
> +			.notifier_call = arm64_hibernate_pm_notify
> +		};
> +
> +		register_pm_notifier(&nb);
> +	}
>  }
>  
>  #ifdef CONFIG_KFENCE
> @@ -1189,11 +1223,6 @@ static void __init map_mem(void)
>  		__map_memblock(start, end, pgprot_tagged(PAGE_KERNEL),
>  			       flags);
>  	}
> -
> -	/* Map the kernel data/bss read-only in the linear map */
> -	__map_memblock(init_end, kernel_end, PAGE_KERNEL_RO, flags);
> -	flush_tlb_kernel_range((unsigned long)lm_alias(__init_end),
> -			       (unsigned long)lm_alias(__bss_stop));
>  }
>  
>  void mark_rodata_ro(void)