Re: [PATCH] virtio_mem: prevent overflow with subblock size

[David Hildenbrand]

On 08.06.20 09:08, Michael S. Tsirkin wrote:
> On Mon, Jun 08, 2020 at 08:58:31AM +0200, David Hildenbrand wrote:
>> On 08.06.20 08:14, Michael S. Tsirkin wrote:
>>> If subblock size is large (e.g. 1G) 32 bit math involving it
>>> can overflow. Rather than try to catch all instances of that,
>>> let's tweak block size to 64 bit.
>>
>> I fail to see where we could actually trigger an overflow. The reported
>> warning looked like a false positive to me.
> 
> 
> So
> 
>     const uint64_t size = count * vm->subblock_size;
> 
> is it unreasonable for count to be 4K with subblock_size being 1M?

virtio_mem_mb_plug_sb() and friends are only called on subblocks
residing within a single Linux memory block. (currently, 128MB .. 2G on
x86-64). A subblock on x86-64 is currently at least 4MB.

So "count * vm->subblock_size" can currently not exceed the Linux memory
block size (in practice, it is max 128MB).

> 
>>>
>>> It ripples through UAPI which is an ABI change, but it's not too late to
>>> make it, and it will allow supporting >4Gbyte blocks while might
>>> become necessary down the road.
>>>
>>
>> This might break cloud-hypervisor, who's already implementing this
>> protocol upstream (ccing Hui).
>> https://github.com/cloud-hypervisor/cloud-hypervisor/blob/master/vm-virtio/src/mem.rs
>>
>> (blocks in the gigabyte range were never the original intention of
>> virtio-mem, but I am not completely opposed to that)
> 
> 
> So in that case, can you code up validation in the probe function?

If we would currently have a "block_size" > Linux memory block size, we
bail out.

virtio_mem_init():

if (vm->device_block_size > memory_block_size_bytes()) {
	dev_err(&vm->vdev->dev,
		"The block size is not supported (too big).\n");
	return -EINVAL;
}

So what's reported can currently not happen. Having that said, changing
"subblock_size" to be an uint64_t is a good cleanup, especially for the
future.




-- 
Thanks,

David / dhildenb