Re: [PATCH] arm64: pageattr: Explicitly bail out when changing permissions for vmalloc_huge mappings

[Ryan Roberts <ryan.roberts@arm.com>]

On 28/03/2025 02:21, Dev Jain wrote:
> arm64 uses apply_to_page_range to change permissions for kernel VA mappings,
> which does not support changing permissions for leaf mappings. This function

I think you mean "block" mappings here? A leaf mapping refers to a page table
entry that maps a piece of memory at any level in the pgtable (i.e. a present
entry that does not map a table).

A block mapping is an Arm ARM term used to mean a leaf mapping at a level other
than the last level (e.g. pmd, pud). A page mapping is an Arm ARM term used to
mean a leaf mapping at the last level (e.g. pte).

> will change permissions until it encounters a leaf mapping, and will bail

block mapping

> out. To avoid this partial change, explicitly disallow changing permissions
> for VM_ALLOW_HUGE_VMAP mappings.

It will also emit a warning. Since there are no reports of this triggering, it
implies that there are currently no cases of code doing a vmalloc_huge()
followed by partial permission change, at least on arm64 (I'm told BPF does do
this on x86 though). But this is a footgun waiting to go off, so let's detect it
early and avoid the possibility of permissions in an intermediate state. (It
might be worth wordsmithing this into the commit log).

> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>

With the commit log fixed up:

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

> ---
>  arch/arm64/mm/pageattr.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 39fd1f7ff02a..8337c88eec69 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -96,7 +96,7 @@ static int change_memory_common(unsigned long addr, int numpages,
>  	 * we are operating on does not result in such splitting.
>  	 *
>  	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
> -	 * Those are guaranteed to consist entirely of page mappings, and
> +	 * Disallow VM_ALLOW_HUGE_VMAP vmalloc mappings so that
>  	 * splitting is never needed.
>  	 *
>  	 * So check whether the [addr, addr + size) interval is entirely
> @@ -105,7 +105,7 @@ static int change_memory_common(unsigned long addr, int numpages,
>  	area = find_vm_area((void *)addr);
>  	if (!area ||
>  	    end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
> -	    !(area->flags & VM_ALLOC))
> +	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
>  		return -EINVAL;
>  
>  	if (!numpages)