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    Subject[PATCH v3 0/6] fs/proc/kcore: don't read offline sections, logically offline pages and hwpoisoned pages
    Date
    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@linux-foundation.org>
    Cc: "Michael S. Tsirkin" <mst@redhat.com>
    Cc: Jason Wang <jasowang@redhat.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Mike Rapoport <rppt@kernel.org>
    Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
    Cc: Oscar Salvador <osalvador@suse.de>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Roman Gushchin <guro@fb.com>
    Cc: Alex Shi <alex.shi@linux.alibaba.com>
    Cc: Steven Price <steven.price@arm.com>
    Cc: Mike Kravetz <mike.kravetz@oracle.com>
    Cc: Aili Yao <yaoaili@kingsoft.com>
    Cc: Jiri Bohac <jbohac@suse.cz>
    Cc: "K. Y. Srinivasan" <kys@microsoft.com>
    Cc: Haiyang Zhang <haiyangz@microsoft.com>
    Cc: Stephen Hemminger <sthemmin@microsoft.com>
    Cc: Wei Liu <wei.liu@kernel.org>
    Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
    Cc: linux-hyperv@vger.kernel.org
    Cc: virtualization@lists.linux-foundation.org
    Cc: linux-fsdevel@vger.kernel.org
    Cc: linux-mm@kvack.org

    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

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