[PATCH v2] arm64/kernel: Always use level 2 or higher for early mappings

[Ryan Roberts]

On 11/03/2025 07:30, 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.
> 
> 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>
> ---
> v2: take the assignment of protval out of the loop again, so that
>     clearing a mapping works as expected wrt the PTE_CONT bit

Ouch, totally missed that. Now that I've looked at the code properly, I wonder
if it is worth supporting CONT_PMD mappings at level 2? That would be 32M for 4K
pages, so is likely to actaually get used. I think it would just be something like:

u64 cmask = (level == 3) ? CONT_PTE_SIZE - 1 :
	   ((level == 2) ? CONT_PMD_SIZE - 1 : U64_MAX) : U64_MAX;

Anyway, probably not worth the complexity for these boot-time tables, so regardless:

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

> 
>  arch/arm64/kernel/pi/map_range.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/pi/map_range.c b/arch/arm64/kernel/pi/map_range.c
> index 2b69e3beeef8..5778697f3062 100644
> --- a/arch/arm64/kernel/pi/map_range.c
> +++ b/arch/arm64/kernel/pi/map_range.c
> @@ -45,12 +45,12 @@ void __init map_range(u64 *pte, u64 start, u64 end, u64 pa, pgprot_t prot,
>  	 * clearing the mapping
>  	 */
>  	if (protval)
> -		protval |= (level < 3) ? PMD_TYPE_SECT : PTE_TYPE_PAGE;
> +		protval |= (level == 2) ? 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.