Re: [patch] kvm: suppress KVM_SET_GSI_ROUTING allocation failure

[Paolo Bonzini]

On 13/02/2018 16:44, Michal Hocko wrote:
> On Tue 13-02-18 16:03:09, Paolo Bonzini wrote:
>> On 13/02/2018 15:48, Michal Hocko wrote:
>>> On Thu 08-02-18 13:35:08, David Rientjes wrote:
>>>> The KVM_SET_GSI_ROUTING ioctl does a vmalloc() of
>>>> sizeof(struct kvm_irq_routing_entry) multiplied by a user-supplied value.
>>>> This can be up to 4096 entries on architectures such as arm64 and s390
>>>> (and the upper bound may be increased on s390 eventually).
>>>>
>>>> This can produce a vmalloc allocation failure warning:
>>>>
>>>> vmalloc: allocation failure: 0 bytes, mode:0x24000c2(GFP_KERNEL|__GFP_HIGHMEM)
>>>
>>> I am not arguing about the kvm change but do we actaully want to warn
>>> for 0 sized allocations? This just doesn't make much sense to me.
>>> In other words don't we want this?
>>>
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>> index 673942094328..c5d832510c54 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -1748,7 +1748,9 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
>>>  	unsigned long real_size = size;
>>>  
>>>  	size = PAGE_ALIGN(size);
>>> -	if (!size || (size >> PAGE_SHIFT) > totalram_pages)
>>> +	if (!size)
>>> +		return NULL;
>>> +	if ((size >> PAGE_SHIFT) > totalram_pages)
>>>  		goto fail;
>>>  
>>>  	area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNINITIALIZED |
>>>
>>
>> There have been quite a few reports of this from syzkaller and generally
>> we've fixed them.  It does seem like a recipe for NULL-pointer
>> dereferences when the size is user-controlled (as in this case).
> 
> We do return NULL for that case regardless the above. The patch just
> doesn't warn. Or do you think it is helpful to warn?

It certainly helps bringing potential issues in the spotlight (through
fuzzing, mostly).

Paolo