Re: [patch] kvm: suppress KVM_SET_GSI_ROUTING allocation failure

[Paolo Bonzini <pbonzini@redhat.com>]

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).

But here I'm actually not sure that the "allocation failure: 0 bytes"
can happen, since we have a check above for "if (routing.nr)", and there
is a check also so that the maximum allocation here is a meager 128 KiB.
 So I'm wondering if this patch is obsolete actually after commit
f8c1b85b2523.  David?

Thanks,

Paolo

Paolo