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

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

On 28/03/25 8:09 pm, Ryan Roberts wrote:
> 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).

Thanks, I will add this.

> 
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
> 
> With the commit log fixed up:
> 
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

Thanks!

> 
>> ---
>>   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)
>