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

["Michael S. Tsirkin" <mst@redhat.com>]

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?

> > 
> > 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?


> > Fixes: 5f1f79bbc9e26 ("virtio-mem: Paravirtualized memory hotplug")
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  drivers/virtio/virtio_mem.c     | 14 +++++++-------
> >  include/uapi/linux/virtio_mem.h |  4 ++--
> >  2 files changed, 9 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> > index 2f357142ea5e..7b1bece8a331 100644
> > --- a/drivers/virtio/virtio_mem.c
> > +++ b/drivers/virtio/virtio_mem.c
> > @@ -77,7 +77,7 @@ struct virtio_mem {
> >  	uint64_t requested_size;
> >  
> >  	/* The device block size (for communicating with the device). */
> > -	uint32_t device_block_size;
> > +	uint64_t device_block_size;
> >  	/* The translated node id. NUMA_NO_NODE in case not specified. */
> >  	int nid;
> >  	/* Physical start address of the memory region. */
> > @@ -86,7 +86,7 @@ struct virtio_mem {
> >  	uint64_t region_size;
> >  
> >  	/* The subblock size. */
> > -	uint32_t subblock_size;
> > +	uint64_t subblock_size;
> >  	/* The number of subblocks per memory block. */
> >  	uint32_t nb_sb_per_mb;
> >  
> > @@ -1698,9 +1698,9 @@ static int virtio_mem_init(struct virtio_mem *vm)
> >  	 * - At least the device block size.
> >  	 * In the worst case, a single subblock per memory block.
> >  	 */
> > -	vm->subblock_size = PAGE_SIZE * 1u << max_t(uint32_t, MAX_ORDER - 1,
> > -						    pageblock_order);
> > -	vm->subblock_size = max_t(uint32_t, vm->device_block_size,
> > +	vm->subblock_size = PAGE_SIZE * 1ul << max_t(uint32_t, MAX_ORDER - 1,
> > +						     pageblock_order);
> > +	vm->subblock_size = max_t(uint64_t, vm->device_block_size,
> >  				  vm->subblock_size);
> >  	vm->nb_sb_per_mb = memory_block_size_bytes() / vm->subblock_size;
> >  
> > @@ -1713,8 +1713,8 @@ static int virtio_mem_init(struct virtio_mem *vm)
> >  
> >  	dev_info(&vm->vdev->dev, "start address: 0x%llx", vm->addr);
> >  	dev_info(&vm->vdev->dev, "region size: 0x%llx", vm->region_size);
> > -	dev_info(&vm->vdev->dev, "device block size: 0x%x",
> > -		 vm->device_block_size);
> > +	dev_info(&vm->vdev->dev, "device block size: 0x%llx",
> > +		 (unsigned long long)vm->device_block_size);
> >  	dev_info(&vm->vdev->dev, "memory block size: 0x%lx",
> >  		 memory_block_size_bytes());
> >  	dev_info(&vm->vdev->dev, "subblock size: 0x%x",
> > diff --git a/include/uapi/linux/virtio_mem.h b/include/uapi/linux/virtio_mem.h
> > index a455c488a995..a9ffe041843c 100644
> > --- a/include/uapi/linux/virtio_mem.h
> > +++ b/include/uapi/linux/virtio_mem.h
> > @@ -185,10 +185,10 @@ struct virtio_mem_resp {
> >  
> >  struct virtio_mem_config {
> >  	/* Block size and alignment. Cannot change. */
> > -	__u32 block_size;
> > +	__u64 block_size;
> >  	/* Valid with VIRTIO_MEM_F_ACPI_PXM. Cannot change. */
> >  	__u16 node_id;
> > -	__u16 padding;
> > +	__u8 padding[6];
> >  	/* Start address of the memory region. Cannot change. */
> >  	__u64 addr;
> >  	/* Region size (maximum). Cannot change. */
> > 
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb