The snd_msndmidi_input_read function in sound/isa/msnd/msnd_midi.c in the Linux kernel through 4.11.7 allows local users to cause a denial of service (over-boundary access) or possibly have unspecified other impact by changing the value of a message queue head pointer between two kernel reads of that value, aka a "double fetch" vulnerability.
https://usn.ubuntu.com/3754-1/
https://github.com/torvalds/linux/commit/20e2b791796bd68816fa115f12be5320de2b8021
https://bugzilla.kernel.org/show_bug.cgi?id=196133