linux-kernel - Re: [PATCH v2] arm64: pageattr: Explicitly bail out when changing permissions for vmalloc

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Message-ID: <36dcb5e3-4406-4763-87f1-8054c2246264@arm.com>
Date: Thu, 3 Apr 2025 15:01:03 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Dev Jain <dev.jain@....com>, catalin.marinas@....com, will@...nel.org
Cc: gshan@...hat.com, rppt@...nel.org, steven.price@....com,
 suzuki.poulose@....com, tianyaxiong@...inos.cn, ardb@...nel.org,
 david@...hat.com, ryan.roberts@....com, urezki@...il.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: pageattr: Explicitly bail out when changing
 permissions for vmalloc_huge mappings



On 4/3/25 10:58, Dev Jain wrote:
> arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings,
> which does not support changing permissions for block mappings. This function
> will change permissions until it encounters a block mapping, and will bail
> out with 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. 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. So,  explicitly disallow changing permissions
> for VM_ALLOW_HUGE_VMAP mappings.
> 
> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
> v1->v2:
>  - Improve changelog, keep mention of page mappings in comment
> 
>  arch/arm64/mm/pageattr.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 39fd1f7ff02a..04d4a8f676db 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -96,8 +96,8 @@ 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
> -	 * splitting is never needed.
> +	 * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
> +	 * mappings are updated and splitting is never needed.
>  	 *
>  	 * So check whether the [addr, addr + size) interval is entirely
>  	 * covered by precisely one VM area that has the VM_ALLOC flag set.
> @@ -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)

LGTM

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>