Re: [PATCH] arm64: mm: Defer read-only remap of data/bss linear alias

[Fuad Tabba]

On Fri, 19 Jun 2026 at 17:40, Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> Fuad reports that in some cases, the KVM init code may apply relocations
> to variables that reside in .data, and does so via the linear map. This
> means that remapping .data read-only beforehand is a bad idea, and
> results in an early boot crash.
>
> These variables in .data are only present when CONFIG_NVHE_EL2_DEBUG or
> CONFIG_NVHE_EL2_TRACING are enabled, which is why it was not spotted in
> testing.
>
> So move the remap to mark_rodata_ro(), which is a reasonable place to
> put this, and ensures that it happens much later during the boot. It
> also means that rodata=off is now taken into account, and so the linear
> alias will remain writable in that case.
>
> Cc: Fuad Tabba <fuad.tabba@xxxxxxxxx>
> Fixes: f2ba877402e5 ("arm64: mm: Map the kernel data/bss read-only in the linear map")
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---

This fixes it, and the changes look good to me, thanks.

Reviewed-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>
Tested-by: Fuad Tabba < fuad.tabba@xxxxxxxxx>

Cheers,
/fuad

>  arch/arm64/mm/mmu.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 9f354971b7e4..1f7eca86b5c1 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1198,11 +1198,6 @@ 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)
> @@ -1221,6 +1216,12 @@ void mark_rodata_ro(void)
>         update_mapping_prot(__pa_symbol(_text), (unsigned long)_text,
>                             (unsigned long)_stext - (unsigned long)_text,
>                             PAGE_KERNEL_RO);
> +
> +       /* Map the kernel data/bss read-only in the linear map */
> +       update_mapping_prot(__pa_symbol(__init_end),
> +                           (unsigned long)lm_alias(__init_end),
> +                           (unsigned long)__bss_stop - (unsigned long)__init_end,
> +                           PAGE_KERNEL_RO);
>  }
>
>  static void __init declare_vma(struct vm_struct *vma,
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>