[PATCH] arm64/kernel: Always use level 2 or higher for early mappings

[Ryan Roberts]

On 10/03/2025 18:30, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> The page table population code in map_range() uses a recursive algorithm
> to create the early mappings of the kernel, the DTB and the ID mapped
> text and data pages, and this fails to take into account that the way
> these page tables may be constructed is not precisely the same at each
> level. In particular, block mappings are not permitted at each level,
> and the code as it exists today might inadvertently create such a
> forbidden block mapping if it were used to map a region of the
> appropriate size and alignment.
> 
> This never happens in practice, given the limited size of the assets
> being mapped by the early boot code. Nonetheless, it would be better if
> this code would behave correctly in all circumstances.

ahha, yes I had a hunch we would be able to guarantee not to hit this condition
in practice. Thanks for the explanation.

> 
> So only permit block mappings at level 2, and page mappings at level 3,
> for any page size, and use table mappings exclusively at all other
> levels. This change should have no impact in practice, but it makes the
> code more robust.
> 
> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
> Reported-by: Ryan Roberts <ryan.roberts at arm.com>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

This looks good to me, and it's D128 ready!

Reviewed-by: Ryan Roberts <ryan.roberts at arm.com>

> ---
>  arch/arm64/kernel/pi/map_range.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/kernel/pi/map_range.c b/arch/arm64/kernel/pi/map_range.c
> index 2b69e3beeef8..025bb9f0aa0b 100644
> --- a/arch/arm64/kernel/pi/map_range.c
> +++ b/arch/arm64/kernel/pi/map_range.c
> @@ -40,17 +40,10 @@ void __init map_range(u64 *pte, u64 start, u64 end, u64 pa, pgprot_t prot,
>  	/* Advance tbl to the entry that covers start */
>  	tbl += (start >> (lshift + PAGE_SHIFT)) % PTRS_PER_PTE;
>  
> -	/*
> -	 * Set the right block/page bits for this level unless we are
> -	 * clearing the mapping
> -	 */
> -	if (protval)
> -		protval |= (level < 3) ? PMD_TYPE_SECT : PTE_TYPE_PAGE;
> -
>  	while (start < end) {
>  		u64 next = min((start | lmask) + 1, PAGE_ALIGN(end));
>  
> -		if (level < 3 && (start | next | pa) & lmask) {
> +		if (level < 2 || (level == 2 && (start | next | pa) & lmask)) {
>  			/*
>  			 * This chunk needs a finer grained mapping. Create a
>  			 * table mapping if necessary and recurse.
> @@ -64,6 +57,14 @@ void __init map_range(u64 *pte, u64 start, u64 end, u64 pa, pgprot_t prot,
>  				  (pte_t *)(__pte_to_phys(*tbl) + va_offset),
>  				  may_use_cont, va_offset);
>  		} else {
> +			/*
> +			 * Set the right block/page bits for this level unless
> +			 * we are clearing the mapping
> +			 */
> +			if (protval)
> +				protval |= (level == 2) ? PMD_TYPE_SECT
> +							: PTE_TYPE_PAGE;
> +
>  			/*
>  			 * Start a contiguous range if start and pa are
>  			 * suitably aligned