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

On Sat, Aug 23, 2025 at 3:25 PM Ard Biesheuvel ardb@kernel.org wrote:

Hi Sam,

On Fri, 22 Aug 2025 at 14:15, Sam Edwards cfsworks@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.

Hi Ard,

I don't believe I can declare the ELF output "inaccurate" per se, since it's the linker's final determination about the state of memory at kernel entry -- including which regions are not the loader's responsibility to initialize (and should therefore be initialized at runtime, e.g. .bss). But, I think I understand your meaning: you would prefer consistent load-time zero-initialization over run-time. I'm open to that approach if that's the consensus here, but it will make `vmlinux` dozens of KBs larger (even though it keeps `Image` the same size).

...

• tramp_pg_dir # Already done, but this patch corrects the size

What is wrong with the size?

On higher-VABIT targets, that memset is overflowing by writing PGD_SIZE bytes despite tramp_pg_dir being only PAGE_SIZE bytes in size. My understanding is that only userspace (TTBR0) PGDs are PGD_SIZE and kernelspace (TTBR1) PGDs like the trampoline mapping are always PAGE_SIZE. Please correct me if I'm wrong; I might be misled by how vmlinux.lds.S is making space for those PGDs. :)

(If you'd like, I can break that one-line change out as a separate patch to apply immediately? It seems like a more critical concern than everything else here.)

Best, Sam

...

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@vger.kernel.org Signed-off-by: Sam Edwards CFSworks@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