[PATCH v3 0/6] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages

[David Hildenbrand]

Looking for places where the kernel might unconditionally read
PageOffline() pages, I stumbled over /proc/kcore; turns out /proc/kcore
needs some more love to not touch some other pages we really don't want to
read -- i.e., hwpoisoned ones.

Examples for PageOffline() pages are pages inflated in a balloon,
memory unplugged via virtio-mem, and partially-present sections in
memory added by the Hyper-V balloon.

When reading pages inflated in a balloon, we essentially produce
unnecessary load in the hypervisor; holes in partially present sections in
case of Hyper-V are not accessible and already were a problem for
/proc/vmcore, fixed in makedumpfile by detecting PageOffline() pages. In
the future, virtio-mem might disallow reading unplugged memory -- marked
as PageOffline() -- in some environments, resulting in undefined behavior
when accessed; therefore, I'm trying to identify and rework all these
(corner) cases.

With this series, there is really only access via /dev/mem, /proc/vmcore
and kdb left after I ripped out /dev/kmem. kdb is an advanced corner-case
use case -- we won't care for now if someone explicitly tries to do nasty
things by reading from/writing to physical addresses we better not touch.
/dev/mem is a use case we won't support for virtio-mem, at least for now,
so we'll simply disallow mapping any virtio-mem memory via /dev/mem next.
/proc/vmcore is really only a problem when dumping the old kernel via
something that's not makedumpfile (read: basically never), however, we'll
try sanitizing that as well in the second kernel in the future.

Tested via kcore_dump:
	https://github.com/schlafwandler/kcore_dump

v2 -> v3:
- "mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize
   setting PageOffline()"
-- Rephrased a comment as suggested by Mike
- Collected acks and rbs

v1 -> v2:
- Dropped "mm: rename and move page_is_poisoned()"
- "fs/proc/kcore: don't read offline sections, logically offline pages ..."
-- Add is_page_hwpoison() in page-flags.h along with a comment
- "mm: introduce page_offline_(begin|end|freeze|thaw) to ..."
-- s/unfreeze/thaw/
-- Add a comment to PageOffline documentation in page-flags.h
- "virtio-mem: use page_offline_(start|end) when setting PageOffline()"
-- Extend patch description
- "fs/proc/kcore: use page_offline_(freeze|thaw)"
-- Simplify freeze/thaw logic
- Collected acks/rbs

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Roman Gushchin <guro@xxxxxx>
Cc: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Cc: Steven Price <steven.price@xxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Aili Yao <yaoaili@xxxxxxxxxxxx>
Cc: Jiri Bohac <jbohac@xxxxxxx>
Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
Cc: Wei Liu <wei.liu@xxxxxxxxxx>
Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
Cc: linux-hyperv@xxxxxxxxxxxxxxx
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx

David Hildenbrand (6):
  fs/proc/kcore: drop KCORE_REMAP and KCORE_OTHER
  fs/proc/kcore: pfn_is_ram check only applies to KCORE_RAM
  fs/proc/kcore: don't read offline sections, logically offline pages
    and hwpoisoned pages
  mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize
    setting PageOffline()
  virtio-mem: use page_offline_(start|end) when setting PageOffline()
  fs/proc/kcore: use page_offline_(freeze|thaw)

 drivers/virtio/virtio_mem.c |  2 ++
 fs/proc/kcore.c             | 67 ++++++++++++++++++++++++++++++-------
 include/linux/kcore.h       |  3 --
 include/linux/page-flags.h  | 22 ++++++++++++
 mm/util.c                   | 40 ++++++++++++++++++++++
 5 files changed, 118 insertions(+), 16 deletions(-)


base-commit: 6efb943b8616ec53a5e444193dccf1af9ad627b5
-- 
2.31.1