Re: [PATCH v3 0/7] Support kdump with LUKS encryption by reusing LUKS volume keys

[Baoquan He]

Please don't add dm-devel@xxxxxxxxxx in the public list because it's a
internal mailing list or aliase. And I got error when replying.

On 04/25/24 at 06:04pm, Coiby Xu wrote:
> LUKS is the standard for Linux disk encryption. Many users choose LUKS
> and in some use cases like Confidential VM it's mandated. With kdump
> enabled, when the 1st kernel crashes, the system could boot into the
> kdump/crash 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,
> 
>  - Kdump kernel may not be able to decrypt the LUKS partition. For some
>    machines, a system administrator may not have a chance to enter the
>    password to decrypt the device in kdump initramfs after the 1st kernel
>    crashes; For cloud confidential VMs, depending on the policy the
>    kdump kernel may not be able to unseal the keys with TPM and the
>    console virtual keyboard is untrusted.
> 
>  - 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. Note if the memory reserved for kdump can't
>    be used by 1st kernel i.e. an user sees ~1300M memory missing in the
>    1st kernel.
> 
> Besides 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 keys again in kdump kernel which seems to be
> redundant work.
> 
> This patch set addresses the above issues by make the LUKS volume keys
> persistent for kdump kernel with the help of cryptsetup's new APIs
> (--link-vk-to-keyring/--volume-key-keyring). Here is the life cycle of
> this kdump copy of LUKS volume keys,
> 
>  1. After the 1st kernel loads the initramfs during boot, systemd
>     use an user-input passphrase or TPM-sealed key to de-crypt the LUKS
>     volume keys and then save the volume keys to specified keyring
>     (using the --link-vk-to-keyring API) and the key will expire within
>     specified time.
> 
>  2. A user space tool (kdump initramfs builder) writes a key description to
>     /sys/kernel/crash_dm_crypt_keys to inform the 1st kernel to record the
>     key while building the kdump initramfs
> 
>  3. The kexec_file_load syscall read the volume keys by recored key
>     descriptions and then save them key to kdump reserved memory and wipe the
>     copy.
> 
>  4. When the 1st kernel crashes and the kdump initramfs is booted, the kdump
>     initramfs asks the kdump kernel to create a user key using the key stored in
>     kdump reserved memory by writing to to /sys/kernel/crash_dm_crypt_keys. Then
>     the LUKS encrypted devide is unlocked with libcryptsetup's
>     --volume-key-keyring API.
> 
>  5. The system gets rebooted to the 1st kernel after dumping vmcore to
>     the LUKS encrypted device is finished
> 
> After libcryptsetup saving the LUKS volume keys to specified keyring,
> whoever takes this should be responsible for the safety of these copies
> of keys. The keys will be saved in the memory area exclusively reserved
> for kdump where even the 1st kernel has no direct access. And further
> more, two additional protections are added,
>  - save the copy randomly in kdump reserved memory as suggested by Jan
>  - clear the _PAGE_PRESENT flag of the page that stores the copy as
>    suggested by Pingfan
> 
> This patch set only supports x86. There will be patches to support other
> architectures once this patch set gets merged.
> 
> v3
>  - Support CPU/memory hot-plugging [Baoquan]
>  - Don't save the keys temporarily to simplify the implementation [Baoquan] 
>  - Support multiple LUKS encrypted volumes
>  - Read logon key instead of user key to improve security [Ondrej]
>  - A kernel config option CRASH_DM_CRYPT for this feature (disabled by default)
>  - Fix warnings found by kernel test robot
>  - Rebase the code onto 6.9.0-rc5+
> 
> v2
>  - work together with libscryptsetup's --link-vk-to-keyring/--volume-key-keyring APIs [Milan and Ondrej]
>  - add the case where console virtual keyboard is untrusted for confidential VM
>  - use dm_crypt_key instead of LUKS volume key [Milan and Eric]
>  - fix some code format issues
>  - don't move "struct kexec_segment" declaration
>  - Rebase the code onto latest Linus tree (6.7.0)
> 
> v1
>  - "Put the luks key handling related to crash_dump out into a separate
>    file kernel/crash_dump_luks.c" [Baoquan]
>  - Put the generic luks handling code before the x86 specific code to
>    make it easier for other arches to follow suit [Baoquan]
>  - Use phys_to_virt instead of "pfn -> page -> vaddr" [Dave Hansen]
>  - Drop the RFC prefix [Dave Young]
>  - Rebase the code onto latest Linus tree (6.4.0-rc4)
> 
> RFC v2
>  - libcryptsetup interacts with the kernel via sysfs instead of "hacking"
>    dm-crypt
>    - to save a kdump copy of the LUKS volume key in 1st kernel
>    - to add a logon key using the copy for libcryptsetup in kdump kernel [Milan]
>    - to avoid the incorrect usage of LUKS master key in dm-crypt [Milan]
>  - save the kdump copy of LUKS volume key randomly [Jan]
>  - mark the kdump copy inaccessible [Pingfan]
>  - Miscellaneous
>    - explain when operations related to the LUKS volume key happen [Jan]
>    - s/master key/volume key/g
>    - use crash_ instead of kexec_ as function prefix
>    - fix commit subject prefixes e.g. "x86, kdump" to x86/crash
> 
> Coiby Xu (7):
>   kexec_file: allow to place kexec_buf randomly
>   crash_dump: make dm crypt keys persist for the kdump kernel
>   crash_dump: store dm keys in kdump reserved memory
>   crash_dump: reuse saved dm crypt keys for CPU/memory hot-plugging
>   crash_dump: retrieve dm crypt keys in kdump kernel
>   x86/crash: pass dm crypt keys to kdump kernel
>   x86/crash: make the page that stores the dm crypt keys inaccessible
> 
>  arch/x86/kernel/crash.c            |  15 +-
>  arch/x86/kernel/kexec-bzimage64.c  |   7 +
>  arch/x86/kernel/machine_kexec_64.c |  21 ++
>  include/linux/crash_core.h         |   9 +-
>  include/linux/crash_dump.h         |   2 +
>  include/linux/kexec.h              |   6 +
>  kernel/Kconfig.kexec               |   8 +
>  kernel/Makefile                    |   1 +
>  kernel/crash_dump_dm_crypt.c       | 319 +++++++++++++++++++++++++++++
>  kernel/kexec_file.c                |  15 ++
>  kernel/ksysfs.c                    |  22 ++
>  11 files changed, 423 insertions(+), 2 deletions(-)
>  create mode 100644 kernel/crash_dump_dm_crypt.c
> 
> -- 
> 2.44.0
>