[PATCH 4/4] arm64: Unmap kernel data/bss entirely from the linear map

[Anshuman Khandual]

On 19/01/26 10:17 PM, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> The linear aliases of the kernel text and rodata are mapped read-only as
> well. Given that the contents of these regions are mostly identical to
> the version in the loadable image, mapping them read-only 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.
> 
> Only when going into hibernation or waking up from it do these regions
> need to be mapped, so take care of this using a PM notifier.

Just curious - why do we need them mapped while going into or coming back
from the hibernation ?

> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm64/mm/mmu.c | 35 ++++++++++++++++++--
>  1 file changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index d978b07ab7b8..7b3ce9cafe64 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>
> @@ -1024,13 +1025,13 @@ static void __init __map_memblock(pgd_t *pgdp, phys_addr_t start,
>  				 prot, early_pgtable_alloc, flags);
>  }
>  
> -static void remap_linear_data_alias(void)
> +static void remap_linear_data_alias(bool unmap)
>  {
>  	extern const u8 __pgdir_start[];
>  
>  	update_mapping_prot(__pa_symbol(__init_end), (unsigned long)lm_alias(__init_end),
>  			    (unsigned long)__pgdir_start - (unsigned long)__init_end,
> -			    PAGE_KERNEL_RO);
> +			    unmap ? __pgprot(0) : PAGE_KERNEL_RO);
>  }
>  
>  void __init remap_linear_kernel_alias(void)
> @@ -1041,7 +1042,7 @@ void __init remap_linear_kernel_alias(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();
> +	remap_linear_data_alias(true);
>  }
>  
>  #ifdef CONFIG_KFENCE
> @@ -2257,3 +2258,31 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey, unsigned long i
>  	return 0;
>  }
>  #endif
> +
> +#ifdef CONFIG_HIBERNATION
> +static int arm64_hibernate_pm_notify(struct notifier_block *nb,
> +				     unsigned long mode, void *unused)
> +{
> +	switch (mode) {
> +	case PM_HIBERNATION_PREPARE:
> +	case PM_RESTORE_PREPARE:
> +		remap_linear_data_alias(false);
> +		break;
> +	case PM_POST_HIBERNATION:
> +	case PM_POST_RESTORE:
> +		remap_linear_data_alias(true);
> +		break;
> +	}
> +	return 0;
> +}
> +
> +static struct notifier_block arm64_hibernate_pm_notifier = {
> +	.notifier_call = arm64_hibernate_pm_notify,
> +};
> +
> +static int arm64_hibernate_register_pm_notifier(void)
> +{
> +	return register_pm_notifier(&arm64_hibernate_pm_notifier);
> +}
> +late_initcall(arm64_hibernate_register_pm_notifier);
> +#endif