Security firm CrowdStrike reports the two kernels have direct access to each other, without hypervisors, just two systems with identical access. It appears this decision was taken deliberately to ensure that the Linux subsystem runs at reasonable performance.
It's a bit of a crazy face-palm moment, really. Who did it not occur to that Windows and Linux apps could be modified by each other, bypassing the patches put in place natively?Note: Bash is not turned on by default in Windows 10 Anniversary Update.
Code injection is just one example of how a Windows program could attack a Linux app. Once the code is injected, if the infected Linux application makes a call back to Windows, it will be trusted and could trigger some proper borkage.