X11 applications are able to get and display in human readable form parts of closed applications, see:
It means applications like skype can silently send much more data, then you expect, even when you run them as separate user.
Asked about this security problem in #gentoo-hardened, #radeon and #dri-devel, summarizing:
0) Currently GPU memory is not sanitized by kernel, nothing enforces and ensures its sanitizing.
1) https://grsecurity.net Linux kernel patches add ability to sanitize all freed memory, but they do not affect GPU memory:
2) The only secure place to clean memory is kernel, because if application or X server crashes, memory will not be erased.
3) Some applications possibly will work slower, so better make sanitizing configurable by adding build and run time kernel options.
4) Most likely this problem affects opencl too.
Freedesktop's Bugzilla instance is EOLed and open bugs are about to be migrated to http://gitlab.freedesktop.org.
To avoid migrating out of date bugs, I am now closing all the bugs that did not see any activity in the past year. If the issue is still happening, please create a new bug in the relevant project at https://gitlab.freedesktop.org/drm (use misc by default).
Sorry about the noise!