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

[Dev Jain <dev.jain@arm.com>]

On 29/03/25 3:16 pm, 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@arm.com>
> 
> I wonder if we want a Fixes: tag here? It's certainly a *latent* bug.

I am struggling to find the commit till which we want to backport. 
Should it be e920722 (arm64: support huge vmalloc mappings)?

> 
>>> ---
>>>   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@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
>>>
>>
>