[RFC 0/4] Support kdump with LUKS encryption by reusing LUKS master key

[Coiby Xu]

With kdump enabled, when kernel crashes, the system could boot into the
kdump kernel and dump the memory image i.e. /proc/vmcore to a specified
target. Currently, when dumping vmcore to a LUKS encrypted device, there
are two problems,
 - for some machines, the user may don't have a chance enter the password
   to decrypt the device after kernel crashes and kdump initrd is loaded
 - LUKS2 by default use the memory-hard Argon2 key derivation function
   which is quite memory-consuming compared to the limited memory reserved
   for kdump. Take Fedora example, by default, only 256M is reserved for
   systems having memory between 4G-64G. With LUKS enabled, ~1300M needs
   to be reserved for kdump.
 
Besides the users (at least for Fedora) usually expect kdump to work out
of the box i.e. no manual password input is needed. And it doesn't make
sense to derivate the master key again in kdump kernel which seems to be
redundant work.

Based on Milan's feedback [1] on Kairui's ideas to support kdump with
LUKS encryption, this patch set addresses the above issues by
 1) first saving the LUKS master key to kexec when opening the encrypted
    device 
 2) then saving the master key to the reserved memory for kdump when 
    loading kdump kernel image.

So the LUKS master key never leaves the kernel space and once the key has
been saved to the reserved memory for kdump, it would be wiped 
immediately. If there is no security concern with this approach or any
other concern, I will drop the following assumptions made for this RFC
version in v1,
 - only x86 is supported
 - there is only one LUKS device for the system

to extend the support to other architectures including POWER, ARM and
s390x and address the case of multiple LUKS devices. Any feedback will be 
appreciated, thanks!

For a proof of concept, I've patched cryptsetup [2] in a quick-and-dirty
way to support a new option "--kdump-kernel-master-key"
and hacked systemd [3]. It works for Fedora 34.

[1] https://yhbt.net/lore/all/e5abd089-3398-fdb4-7991-0019be434b79@gmail.com/
[2] https://gitlab.com/coxu/cryptsetup/-/commit/ee54bb15445da0bc3f9155a7227a9799da4dac20
[3] https://github.com/coiby/systemd/tree/reuse_kdump_master_key

Coiby Xu (4):
  kexec, dm-crypt: receive LUKS master key from dm-crypt and pass it to
    kdump
  kdump, x86: pass the LUKS master key to kdump kernel using a kernel
    command line parameter luksmasterkey
  crash_dump: retrieve LUKS master key in kdump kernel
  dm-crypt: reuse LUKS master key in kdump kernel

 arch/x86/include/asm/crash.h      |  1 +
 arch/x86/kernel/crash.c           | 42 ++++++++++++++++++-
 arch/x86/kernel/kexec-bzimage64.c |  7 ++++
 drivers/md/dm-crypt.c             | 26 +++++++++---
 include/linux/crash_dump.h        |  4 ++
 include/linux/kexec.h             |  7 ++++
 kernel/crash_dump.c               | 69 +++++++++++++++++++++++++++++++
 kernel/kexec_core.c               | 66 +++++++++++++++++++++++++++++
 8 files changed, 215 insertions(+), 7 deletions(-)

-- 
2.34.1