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@xxxxxxxxxx>
