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From:   David Hildenbrand <david@redhat.com>
To:     linux-kernel@vger.kernel.org
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Subject: [PATCH v1 0/3] drivers/char: remove /dev/kmem for good
Date:   Wed, 24 Mar 2021 11:23:48 +0100
Message-Id: <20210324102351.6932-1-david@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Let's remove /dev/kmem, which is unused and obsolete. Description from
patch #1:

"
Exploring /dev/kmem and /dev/mem in the context of memory hot(un)plug and
memory ballooning, I started questioning the existance of /dev/kmem.

Comparing it with the /proc/kcore implementation, it does not seem to be
able to deal with things like
a) Pages unmapped from the direct mapping (e.g., to be used by secretmem)
  -> kern_addr_valid(). virt_addr_valid() is not sufficient.
b) Special cases like gart aperture memory that is not to be touched
  -> mem_pfn_is_ram()
Unless I am missing something, it's at least broken in some cases and might
fault/crash the machine.

Looks like its existance has been questioned before in 2005 and 2010
[1], after ~11 additional years, it might make sense to revive the
discussion.

CONFIG_DEVKMEM is only enabled in a single defconfig (on purpose or by
mistake?). All distributions disable it: in Ubuntu it has been disabled
for more than 10 years, in Debian since 2.6.31, in Fedora at least starting
with FC3, in RHEL starting with RHEL4, in SUSE starting from 15sp2, and
OpenSUSE has it disabled as well.

1) /dev/kmem was popular for rootkits [2] before it got disabled
   basically everywhere. Ubuntu documents [3] "There is no modern user of
   /dev/kmem any more beyond attackers using it to load kernel rootkits.".
   RHEL documents in a BZ [5] "it served no practical purpose other than to
   serve as a potential security problem or to enable binary module drivers
   to access structures/functions they shouldn't be touching"

2) /proc/kcore is a decent interface to have a controlled way to read
   kernel memory for debugging puposes. (will need some extensions to
   deal with memory offlining/unplug, memory ballooning, and poisoned
   pages, though)

3) It might be useful for corner case debugging [1]. KDB/KGDB might be a
   better fit, especially, to write random memory; harder to shoot
   yourself into the foot.

4) "Kernel Memory Editor" [4] hasn't seen any updates since 2000 and seems
   to be incompatible with 64bit [1]. For educational purposes,
   /proc/kcore might be used to monitor value updates -- or older
   kernels can be used.

5) It's broken on arm64, and therefore, completely disabled there.

Looks like it's essentially unused and has been replaced by better
suited interfaces for individual tasks (/proc/kcore, KDB/KGDB). Let's
just remove it.

[1] https://lwn.net/Articles/147901/
[2] https://www.linuxjournal.com/article/10505
[3] https://wiki.ubuntu.com/Security/Features#A.2Fdev.2Fkmem_disabled
[4] https://sourceforge.net/projects/kme/
[5] https://bugzilla.redhat.com/show_bug.cgi?id=154796
"

RFC -> v1:
- "drivers/char: remove /dev/kmem for good"
-- Add more details to patch description regarding distributions
- "mm/vmalloc: remove vwrite()"
-- Also remove the nommu variant
- Compile-tested on more archs/configs

David Hildenbrand (3):
  drivers/char: remove /dev/kmem for good
  mm: remove xlate_dev_kmem_ptr()
  mm/vmalloc: remove vwrite()

 Documentation/admin-guide/devices.txt     |   2 +-
 arch/alpha/include/asm/io.h               |   5 -
 arch/arm/configs/dove_defconfig           |   1 -
 arch/arm/configs/magician_defconfig       |   1 -
 arch/arm/configs/moxart_defconfig         |   1 -
 arch/arm/configs/mps2_defconfig           |   1 -
 arch/arm/configs/mvebu_v5_defconfig       |   1 -
 arch/arm/configs/xcep_defconfig           |   1 -
 arch/arm/include/asm/io.h                 |   5 -
 arch/h8300/configs/edosk2674_defconfig    |   1 -
 arch/h8300/configs/h8300h-sim_defconfig   |   1 -
 arch/h8300/configs/h8s-sim_defconfig      |   1 -
 arch/hexagon/configs/comet_defconfig      |   1 -
 arch/hexagon/include/asm/io.h             |   1 -
 arch/ia64/include/asm/io.h                |   1 -
 arch/ia64/include/asm/uaccess.h           |  18 --
 arch/m68k/configs/amcore_defconfig        |   1 -
 arch/m68k/include/asm/io_mm.h             |   5 -
 arch/mips/include/asm/io.h                |   5 -
 arch/openrisc/configs/or1ksim_defconfig   |   1 -
 arch/parisc/include/asm/io.h              |   5 -
 arch/powerpc/include/asm/io.h             |   5 -
 arch/s390/include/asm/io.h                |   5 -
 arch/sh/configs/edosk7705_defconfig       |   1 -
 arch/sh/configs/se7206_defconfig          |   1 -
 arch/sh/configs/sh2007_defconfig          |   1 -
 arch/sh/configs/sh7724_generic_defconfig  |   1 -
 arch/sh/configs/sh7770_generic_defconfig  |   1 -
 arch/sh/configs/sh7785lcr_32bit_defconfig |   1 -
 arch/sh/include/asm/io.h                  |   5 -
 arch/sparc/configs/sparc64_defconfig      |   1 -
 arch/sparc/include/asm/io_64.h            |   5 -
 arch/xtensa/configs/xip_kc705_defconfig   |   1 -
 drivers/char/Kconfig                      |  10 -
 drivers/char/mem.c                        | 231 ----------------------
 include/asm-generic/io.h                  |  11 --
 include/linux/fs.h                        |   2 +-
 include/linux/vmalloc.h                   |   3 +-
 kernel/configs/android-base.config        |   1 -
 mm/ksm.c                                  |   2 +-
 mm/nommu.c                                |  10 -
 mm/vmalloc.c                              | 118 +----------
 42 files changed, 6 insertions(+), 469 deletions(-)


base-commit: 0d02ec6b3136c73c09e7859f0d0e4e2c4c07b49b
-- 
2.29.2