Software installed and run as a non-privileged user may conduct improper GPU system calls to subvert GPU HW to write to arbitrary physical memory pages.
Kernel software installed and running inside a Guest VM may exploit memory shared with the GPU Firmware to write data outside the Guest's virtualised GPU memory.
A GPU kernel can read sensitive data from another GPU kernel (even from another user or app) through an optimized GPU memory region called _local memory_ on various architectures.