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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
