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

[Baoquan He]

Hi Andrew,

On 02/11/25 at 06:25pm, Baoquan He wrote:
> 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.

Could you pick this patchset into your tree since no conern from other
reviewers?

Thanks
Baoquan

> 
> This v8 looks good to me, thanks for the great effort, Coiby.
> 
> Acked-by: Baoquan He <bhe@xxxxxxxxxx>
>