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vmspawn: deliver credentials via initrd cpio under SEV-SNP

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Previously, --load-credential / --set-credential were rejected outright
under --coco=sev-snp because the SMBIOS type-11 transport isn't covered
by the launch measurement. PID1 wouldn't have accepted those credentials
anyway (import_credentials_smbios() refuses any SMBIOS-sourced credentials
under a confidential VM).

Instead, when SNP is in use and credentials are present, synthesize a
newc cpio archive containing each credential at
.extra/system_credentials/<id>.cred and append it to the initrd list.
The existing merge_initrds() path then concatenates it into the single
initrd file QEMU loads, which kernel-hashes=on covers in the SEV-SNP
launch digest. PID1's import_credentials_boot() picks them up from the
trusted /.extra/system_credentials/ path and routes them to the @system
bucket, so units can consume them via LoadCredential= unchanged.

Direct kernel boot (--linux=) is already required under SNP, so the
initrd is always under our control here. The cpio synthesis happens
after all internal machine_credential_add()/machine_credential_load()
call sites so the archive captures the complete credential set (journal
forwarding, vmm.notify_socket, ssh ephemeral keys, etc.).

The cpio path is intentionally scoped to SNP: it requires a guest PID1
that knows about /.extra/system_credentials/, and we don't want to
regress credential delivery for non-CoCo guests running older systemd
versions in the guest. Consider switching when the new path is widely
available.

Signed-off-by: Paul Meyer <katexochen0@gmail.com>
This commit is contained in:
Paul Meyer
2026-06-24 12:47:42 +02:00
parent 62fde7bc06
commit 76b62b0435
1 changed files with 50 additions and 27 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/vmspawn/vmspawn.c
+50 -27
View File
@@ -46,6 +46,7 @@
#include "hostname-setup.h"
#include "hostname-util.h"
#include "id128-util.h"
#include "initrd-cpio.h"
#include "kernel-image.h"
#include "log.h"
#include "machine-bind-user.h"
@@ -57,8 +58,8 @@
#include "namespace-util.h"
#include "netif-util.h"
#include "nsresource.h"
#include "osc-context.h"
#include "options.h"
#include "osc-context.h"
#include "pager.h"
#include "parse-argument.h"
#include "parse-util.h"
@@ -3424,26 +3425,6 @@ static int run_virtual_machine(int kvm_device_fd, int vhost_device_fd) {
}
}
char *initrd = NULL;
_cleanup_(rm_rf_physical_and_freep) char *merged_initrd = NULL;
size_t n_initrds = strv_length(arg_initrds);
if (n_initrds == 1)
initrd = arg_initrds[0];
else if (n_initrds > 1) {
r = merge_initrds(&merged_initrd);
if (r < 0)
return r;
initrd = merged_initrd;
}
if (initrd) {
r = strv_extend_many(&cmdline, "-initrd", initrd);
if (r < 0)
return log_oom();
}
if (arg_forward_journal) {
_cleanup_free_ char *listen_address = NULL;
if (asprintf(&listen_address, "vsock:2:%u", child_cid) < 0)
@@ -3545,9 +3526,54 @@ static int run_virtual_machine(int kvm_device_fd, int vhost_device_fd) {
return log_error_errno(r, "Failed to add VSOCK credential: %m");
}
r = cmdline_add_credentials(&cmdline, smbios_dir_fd, smbios_dir);
if (r < 0)
return r;
/* Under --coco=sev-snp the SMBIOS and fw_cfg channels normally used to deliver credentials are
* not covered by the launch measurement and are silently discarded by the guest PID1 in
* confidential VMs. Instead, package credentials into a cpio archive appended to the initrd
* (mirroring what systemd-stub does for ESP credentials) so they enter the launch measurement
* via QEMU's "kernel-hashes=on". The new initrd path requires a guest PID1 that knows about
* /.extra/system_credentials/, so we keep this scoped to SNP for now. Non-CoCo guests
* continue to use the SMBIOS path below, which works with older systemd versions too.
* Must run after all credential-mutating calls above so the cpio captures the complete set. */
bool use_initrd_cpio = arg_confidential_computing == COCO_AMD_SEV_SNP &&
arg_credentials.n_credentials > 0;
_cleanup_(unlink_and_freep) char *credentials_cpio_path = NULL;
if (use_initrd_cpio) {
r = initrd_cpio_credentials_to_tempfile(&arg_credentials, &credentials_cpio_path);
if (r < 0)
return r;
r = strv_extend(&arg_initrds, credentials_cpio_path);
if (r < 0)
return log_oom();
}
char *initrd = NULL;
_cleanup_(rm_rf_physical_and_freep) char *merged_initrd = NULL;
size_t n_initrds = strv_length(arg_initrds);
if (n_initrds == 1)
initrd = arg_initrds[0];
else if (n_initrds > 1) {
r = merge_initrds(&merged_initrd);
if (r < 0)
return r;
initrd = merged_initrd;
}
if (initrd) {
r = strv_extend_many(&cmdline, "-initrd", initrd);
if (r < 0)
return log_oom();
}
/* Under SNP, credentials flow via the initrd cpio above. For everyone else, use the
* SMBIOS/fw_cfg/cmdline path. */
if (!use_initrd_cpio) {
r = cmdline_add_credentials(&cmdline, smbios_dir_fd, smbios_dir);
if (r < 0)
return r;
}
r = cmdline_add_kernel_cmdline(&cmdline, smbios_dir_fd, smbios_dir);
if (r < 0)
@@ -4115,9 +4141,6 @@ static int verify_arguments(void) {
if (set_contains(arg_firmware_features_include, "secure-boot"))
return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
"--secure-boot=yes cannot be combined with --coco.");
if (arg_credentials.n_credentials != 0)
return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
"SMBIOS credentials aren't trusted by the confidential computing guest and will be rejected.");
if (arg_tpm > 0)
log_warning("TPM can't be trusted by the confidential computing guest");
/* kernel-hashes=on only covers what QEMU itself loads via -kernel/-initrd/-append.