Re: [PATCH v8 0/7] Support kdump with LUKS encryption by reusing LUKS volume keys

[Baoquan He <bhe@redhat.com>]

On 02/07/25 at 04:08pm, Coiby Xu wrote:
> LUKS is the standard for Linux disk encryption, widely adopted by users,
> and in some cases, such as Confidential VMs, it is a requirement. With 
> kdump enabled, when the first kernel crashes, the system can boot into
> the kdump/crash kernel to dump the memory image (i.e., /proc/vmcore) 
> to a specified target. However, there are two challenges when dumping
> vmcore to a LUKS-encrypted device:
> 
>  - 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 or custom crashkernel value 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 making 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
> the kdump copies of LUKS volume keys,
> 
>  1. After the 1st kernel loads the initramfs during boot, systemd
>     use an user-input passphrase to de-crypt the LUKS volume keys
>     or TPM-sealed key 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 loader like kdump-utils) create
>     key items inside /sys/kernel/config/crash_dm_crypt_keys to inform
>     the 1st kernel which keys are needed.
> 
>  3. When the kdump initramfs is loaded by the kexec_file_load
>     syscall, the 1st kernel will iterate created key items, save the
>     keys to kdump reserved memory.
> 
>  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 yes to
>     /sys/kernel/crash_dm_crypt_keys/restore. Then the LUKS encrypted
>     device 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.

This v8 looks good to me, thanks for the great effort, Coiby.

Acked-by: Baoquan He <bhe@redhat.com>