Microsoft contributes Hyper-V drivers to the kernel

Microsoft would put itself in a particularly tenuous legal position and likely destroy their ability to sell software to companies with legal departments if they distributed code under some license and then sued people for patent infringement for using that code under that license. Can any Microsoft software be used at all without violating a Microsoft patent which the purchaser has not explicitly granted a license to? (Particularly as the licenses for new software may be written too soon to have explicit references to new patents the software uses.)