This is a kernel bug, introduced by one of these patches applied to xenial:
commit d823dfc03906fd278ae6f48219c4ed53faee6f9d Author: Al Viro <email address hidden> Date: Thu Jun 27 20:16:00 2019 +0200
bio_copy_from_iter(): get rid of copying iov_iter
commit aceb27a7de5fc65a3fc70ccac344f33099e8a4dc Author: Keith Busch <email address hidden> Date: Thu Jun 27 20:16:00 2019 +0200
block: Clear kernel memory before copying to user
commit a713ff5b13e8fd9e331fd84232806316aa9e9b2a Author: Keith Busch <email address hidden> Date: Thu Jun 27 20:16:00 2019 +0200
block/bio: Do not zero user pages
This is a kernel bug, introduced by one of these patches applied to xenial:
commit d823dfc03906fd2 78ae6f48219c4ed 53faee6f9d
Author: Al Viro <email address hidden>
Date: Thu Jun 27 20:16:00 2019 +0200
bio_ copy_from_ iter(): get rid of copying iov_iter
commit aceb27a7de5fc65 a3fc70ccac344f3 3099e8a4dc
Author: Keith Busch <email address hidden>
Date: Thu Jun 27 20:16:00 2019 +0200
block: Clear kernel memory before copying to user
commit a713ff5b13e8fd9 e331fd842328063 16aa9e9b2a
Author: Keith Busch <email address hidden>
Date: Thu Jun 27 20:16:00 2019 +0200
block/bio: Do not zero user pages