[PATCH] arm64/boot: Zero-initialize idmap PGDs before use

[Ard Biesheuvel]

Hi Sam,

On Fri, 22 Aug 2025 at 14:15, Sam Edwards <cfsworks at gmail.com> wrote:
>
> In early boot, Linux creates identity virtual->physical address mappings
> so that it can enable the MMU before full memory management is ready.
> To ensure some available physical memory to back these structures,
> vmlinux.lds reserves some space (and defines marker symbols) in the
> middle of the kernel image. However, because they are defined outside of
> PROGBITS sections, they aren't pre-initialized -- at least as far as ELF
> is concerned.
>
> In the typical case, this isn't actually a problem: the boot image is
> prepared with objcopy, which zero-fills the gaps, so these structures
> are incidentally zero-initialized (an all-zeroes entry is considered
> absent, so zero-initialization is appropriate).
>
> However, that is just a happy accident: the `vmlinux` ELF output
> authoritatively represents the state of memory at entry. If the ELF
> says a region of memory isn't initialized, we must treat it as
> uninitialized. Indeed, certain bootloaders (e.g. Broadcom CFE) ingest
> the ELF directly -- sidestepping the objcopy-produced image entirely --
> and therefore do not initialize the gaps. This results in the early boot
> code crashing when it attempts to create identity mappings.
>
> Therefore, add boot-time zero-initialization for the following:
> - __pi_init_idmap_pg_dir..__pi_init_idmap_pg_end
> - idmap_pg_dir
> - reserved_pg_dir

I don't think this is the right approach.

If the ELF representation is inaccurate, it should be fixed, and this
should be achievable without impacting the binary image at all.

> - tramp_pg_dir # Already done, but this patch corrects the size
>

What is wrong with the size?

> Note, swapper_pg_dir is already initialized (by copy from idmap_pg_dir)
> before use, so this patch does not need to address it.
>
> Cc: stable at vger.kernel.org
> Signed-off-by: Sam Edwards <CFSworks at gmail.com>
> ---
>  arch/arm64/kernel/head.S | 12 ++++++++++++
>  arch/arm64/mm/mmu.c      |  3 ++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index ca04b338cb0d..0c3be11d0006 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -86,6 +86,18 @@ SYM_CODE_START(primary_entry)
>         bl      record_mmu_state
>         bl      preserve_boot_args
>
> +       adrp    x0, reserved_pg_dir
> +       add     x1, x0, #PAGE_SIZE
> +0:     str     xzr, [x0], 8
> +       cmp     x0, x1
> +       b.lo    0b
> +
> +       adrp    x0, __pi_init_idmap_pg_dir
> +       adrp    x1, __pi_init_idmap_pg_end
> +1:     str     xzr, [x0], 8
> +       cmp     x0, x1
> +       b.lo    1b
> +
>         adrp    x1, early_init_stack
>         mov     sp, x1
>         mov     x29, xzr
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 34e5d78af076..aaf823565a65 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -761,7 +761,7 @@ static int __init map_entry_trampoline(void)
>         pgprot_val(prot) &= ~PTE_NG;
>
>         /* Map only the text into the trampoline page table */
> -       memset(tramp_pg_dir, 0, PGD_SIZE);
> +       memset(tramp_pg_dir, 0, PAGE_SIZE);
>         __create_pgd_mapping(tramp_pg_dir, pa_start, TRAMP_VALIAS,
>                              entry_tramp_text_size(), prot,
>                              pgd_pgtable_alloc_init_mm, NO_BLOCK_MAPPINGS);
> @@ -806,6 +806,7 @@ static void __init create_idmap(void)
>         u64 end   = __pa_symbol(__idmap_text_end);
>         u64 ptep  = __pa_symbol(idmap_ptes);
>
> +       memset(idmap_pg_dir, 0, PAGE_SIZE);
>         __pi_map_range(&ptep, start, end, start, PAGE_KERNEL_ROX,
>                        IDMAP_ROOT_LEVEL, (pte_t *)idmap_pg_dir, false,
>                        __phys_to_virt(ptep) - ptep);
> --
> 2.49.1
>