Re: [PATCH v2] arm64: pageattr: Explicitly bail out when changing permissions for vmalloc_huge mappings

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

On 04/04/2025 13:57, David Hildenbrand wrote:
> On 03.04.25 07:28, 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@arm.com>
>> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>> Signed-off-by: Dev Jain <dev.jain@arm.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)
> 
> Makes sense to me. Whenever required, we can improve the checks (or even try
> supporting splitting).

Yes that's the plan; I'd like to get to the point where we can do huge vmalloc
mappings by default on systems that support BBML2, then split mappings when needed.


> 
> Acked-by: David Hildenbrand <david@redhat.com>
>