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

[Dev Jain]

On 30/03/25 1:02 pm, Mike Rapoport wrote:
> On Sat, Mar 29, 2025 at 09:46:56AM +0000, Ryan Roberts wrote:
>> On 28/03/2025 18:50, Mike Rapoport wrote:
>>> On Fri, Mar 28, 2025 at 11:51:03AM +0530, Dev Jain wrote:
>>>> arm64 uses apply_to_page_range to change permissions for kernel VA mappings,
>>>
>>>                                                       for vmalloc mappings ^
>>>
>>> arm64 does not allow changing permissions to any VA address right now.
>>>
>>>> which does not support changing permissions for leaf mappings. This function
>>>> will change permissions until it encounters a leaf mapping, and will bail
>>>> out. To avoid this partial change, explicitly disallow changing permissions
>>>> for VM_ALLOW_HUGE_VMAP mappings.
>>>>
>>>> Signed-off-by: Dev Jain <dev.jain at arm.com>
>>
>> I wonder if we want a Fixes: tag here? It's certainly a *latent* bug.
> 
> We have only a few places that use vmalloc_huge() or VM_ALLOW_HUGE_VMAP and
> if there was a code that plays permission games on these allocations, x86
> set_memory would blow up immediately, so I don't think Fixes: is needed
> here.

But I think x86 can handle this (split_large_page() in 
__change_page_attr()) ?

>   
>>>> ---
>>>>   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
>>>
>>> I'd keep mention of page mappings in the comment, e.g
>>>
>>> 	* Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
>>> 	* mappings are updated and splitting is never needed.
>>>
>>> With this and changelog updates Ryan asked for
>>>
>>> Reviewed-by: Mike Rapoport (Microsoft) <rppt at kernel.org>
>>>
>>>
>>>>   	 * 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)
>>>> -- 
>>>> 2.30.2
>>>>
>>>
>>
>