Thomas Hellstrom of VMware wrote on the Linux kernel mailing list, "as was discussed a while ago, there are some serious security flaws with the current drm master model, that allows a user that had previous access or current access to an X server terminal to access the GPU memory of the active X server, without being authenticated to the X server and thereby also access other user's secret information."
Security-related bugs within the open-source Linux graphics drivers and X.Org stack are sadly not a rare occurrence. The X.Org security has been characterized by experts as being a disaster that is worse than it looks. Improvements have been made and security vulnerabilities addressed, including some bugs that date back two decades.
Hellstrom is looking to improve the security around DRM drivers in situations like a user using an X.Org Server and locking the screen while an untrusted user could VT switch, open a DRM connection, and guess GEM handle names of the switched-away X Server to dump the video memory objects. There's no DRM driver currently that is immune against three different security flaws noted by Thomas.
In Hellstrom's email he lays out several possibilities for kernel DRM developers to address these security flaws. We'll see in future kernel releases what's done to improve the situation.