Re: [PATCH v7 15/15] arm64: mm: Unmap kernel data/bss entirely from the linear map

[Marek Szyprowski <m.szyprowski@samsung.com>]

Dear All,

On 29.05.2026 17:02, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
>
> The linear aliases of the kernel text and rodata are also mapped
> read-only in the linear map. 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.
>
> Doing so is required because pages covering the kernel image are marked
> as PageReserved, and therefore disregarded for snapshotting by the
> hibernate logic unless they are mapped.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
This commit landed in yesterday's linux-next as commit 63e0b6a5b693
("arm64: mm: Unmap kernel data/bss entirely from the linear map").
In my tests I found that it breaks booting of RaspberryPi3 and
RaspberryPi4 boards with the following kernel panic:

kvm [1]: nv: 570 coarse grained trap handlers
kvm [1]: nv: 710 fine grained trap handlers
kvm [1]: IPA Size Limit: 40 bits
Unable to handle kernel paging request at virtual address ffff000003a23000
Mem abort info:
  ESR = 0x0000000096000147
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x07: level 3 translation fault
Data abort info:
  ISV = 0, ISS = 0x00000147, ISS2 = 0x00000000
  CM = 1, WnR = 1, TnD = 0, TagAccess = 0
  GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
swapper pgtable: 4k pages, 48-bit VAs, pgdp=0000000002609000
[ffff000003a23000] pgd=0000000000000000, p4d=180000003b3ff403, pud=180000003b3fe403, pmd=180000003b3e6403, pte=00e8000003a23f06
Internal error: Oops: 0000000096000147 [#1]  SMP
Modules linked in:
CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.1.0-rc1+ #16768 PREEMPT
Hardware name: Raspberry Pi 3 Model B (DT)
pstate: 80000005 (Nzcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : dcache_clean_inval_poc+0x24/0x48
lr : kvm_arm_init+0xa8c/0x165c
sp : ffff8000844bbd00
...
Call trace:
 dcache_clean_inval_poc+0x24/0x48 (P)
 do_one_initcall+0x68/0x4f4
 kernel_init_freeable+0x24c/0x360
 kernel_init+0x24/0x1dc
 ret_from_fork+0x10/0x20
Code: 9ac32042 d1000443 8a230000 d503201f (d50b7e20)
---[ end trace 0000000000000000 ]---
Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
SMP: stopping secondary CPUs
Kernel Offset: disabled
CPU features: 0x00000000,03000008,00040000,0400421b
Memory Limit: none
---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---



> ---
>  arch/arm64/mm/mmu.c | 45 ++++++++++++++++++--
>  1 file changed, 41 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 7b18dc2f1721..07a6fa210171 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 mark_linear_data_alias_valid(bool valid)
> +{
> +	set_memory_valid((unsigned long)lm_alias(__init_end),
> +			 (unsigned long)(__bss_stop - __init_end) / PAGE_SIZE,
> +			 valid);
> +}
> +
> +static int arm64_hibernate_pm_notify(struct notifier_block *nb,
> +				     unsigned long mode, void *unused)
> +{
> +	switch (mode) {
> +	default:
> +		break;
> +	case PM_POST_HIBERNATION:
> +		mark_linear_data_alias_valid(false);
> +		break;
> +	case PM_HIBERNATION_PREPARE:
> +		mark_linear_data_alias_valid(true);
> +		break;
> +	}
> +	return 0;
> +}
> +
>  void __init mark_linear_text_alias_ro(void)
>  {
>  	/*
> @@ -1064,6 +1088,21 @@ 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);
> +
> +	/*
> +	 * Register a PM notifier to remap the linear alias of data/bss as
> +	 * valid read-only before hibernation. This is needed because the
> +	 * snapshot logic disregards PageReserved pages (such as the ones
> +	 * covering the kernel image) unless they are mapped in the linear
> +	 * map.
> +	 */
> +	if (IS_ENABLED(CONFIG_HIBERNATION)) {
> +		static struct notifier_block nb = {
> +			.notifier_call = arm64_hibernate_pm_notify
> +		};
> +
> +		register_pm_notifier(&nb);
> +	}
>  }
>  
>  #ifdef CONFIG_KFENCE
> @@ -1193,10 +1232,8 @@ static void __init map_mem(void)
>  			       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));
> +	/* Map the kernel data/bss as invalid in the linear map */
> +	mark_linear_data_alias_valid(false);
>  }
>  
>  void mark_rodata_ro(void)

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland