Linux Kernel: Missing access_ok() checks in IOCTL function (gpu/drm/i915 Driver)

[Timothy Michaud <tmm08a () acu edu>]

NOTE: I have requested a CVE identifier, and I'm sending this message, to
make tracking of the fix easier; however, to avoid missing security fixes
without CVE identifiers, you should *NOT* be cherry-picking a specific
patch in response to a notification about a kernel security bug.

Due to a lack of "access_ok()" checks in i915_gem_execbuffer2_ioctl[1], it
is possible to escalate privileges similar to the waitid vulnerability[2]

This is CVE-2018-20669

[1] -
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=594cc251fdd0d231d342d88b2fdff4bc42fb0690
[2] - https://salls.github.io/Linux-Kernel-CVE-2017-5123/