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sd-stub: measure sysext images picked up by sd-stub into PCR 13

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Let's grab another so far unused PCR, and measure all sysext images into
it that we load from the ESP. Note that this is possibly partly redundant,
since sysext images should have dm-verity enabled, and that is hooked up
to IMA. However, measuring this explicitly has the benefit that we can
measure filenames too, easily, and that all without need for IMA or
anything like that.

This means: when booting a unified sd-stub kernel through sd-boot we'll
now have:

1. PCR 11: unified kernel image payload (i.e. kernel, initrd, boot
   splash, dtb, osrelease)

2. PCR 12: kernel command line (i.e. the one embedded in the image, plus
   optionally an overriden one) + any credential files picked up by
   sd-stub

3. PCR 13: sysext images picked up by sd-stub

And each of these three PCRs should carry just the above, and start from
zero, thus be pre-calculatable.

Thus, all components and parameters of the OS boot process (i.e.
everything after the boot loader) is now nicely pre-calculable.

NOTE: this actually replaces previous measuring of the syext images into
PCR 4. I added this back in 845707aae23b3129db635604edb95c4048a5922a,
following the train of thought, that sysext images for the initrd should
be measured like the initrd itself they are for, and according to my
thinking that would be a unified kernel which is measured by firmware
into PCR 4 like any other UEFI executables.

However, I think we should depart from that idea. First and foremost
that makes it harder to pre-calculate PCR 4 (since we actually measured
quite incompatible records to the TPM event log), but also I think
there's great value in being able to write policies that bind to the
used sysexts independently of the earlier boot chain (i.e. shim, boot
loader, unified kernel), hence a separate PCR makes more sense.

Strictly speaking, this is a compatibility break, but I think one we can
get away with, simply because the initrd sysext images are currently not
picked up by systemd-sysext yet in the initrd, and because of that we
can be reasonably sure noone uses this yet, and hence relies on the PCR
register used. Hence, let's clean this up before people actually do
start relying on this.
This commit is contained in:
Lennart Poettering 2022-07-26 11:35:57 +02:00
parent 16700cb85a
commit de7ad6d4f4
4 changed files with 34 additions and 17 deletions
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5
man/systemd-cryptenroll.xml
View File

@ -306,6 +306,11 @@
<entry><citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> measures any specified kernel command line into this PCR. <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> measures any manually specified kernel command line (i.e. a kernel command line that overrides the one embedded in the unified PE image) and loaded credentials into this PCR. (Note that if <command>systemd-boot</command> and <command>systemd-stub</command> are used in combination the command line might be measured twice!)</entry>
</row>
<row>
<entry>13</entry>
<entry><citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> measures any <citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> images it loads and passed to the booted kernel into this PCR.</entry>
</row>
<row>
<entry>14</entry>
<entry>The shim project measures its "MOK" certificates and hashes into this PCR.</entry>

13
man/systemd-stub.xml
View File

@ -115,7 +115,7 @@
images to the initrd. See
<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
details on system extension images. The generated <command>cpio</command> archive containing these
system extension images is measured into TPM PCR 4 (if a TPM is present).</para></listitem>
system extension images is measured into TPM PCR 13 (if a TPM is present).</para></listitem>
<listitem><para>Files <filename>/loader/credentials/*.cred</filename> are packed up in a
<command>cpio</command> archive and placed in the <filename>/.extra/global_credentials/</filename>
@ -197,7 +197,7 @@
<row>
<entry>System Extensions (synthesized initrd from companion files)</entry>
<entry>4 + 9</entry>
<entry>9 + 13</entry>
</row>
</tbody>
</tgroup>
@ -263,6 +263,15 @@
formatted as decimal ASCII string (i.e. <literal>12</literal>). This variable is set if a measurement
was successfully completed, and remains unset otherwise.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>StubPcrInitRDSysExts</varname></term>
<listitem><para>The PCR register index the systemd extensions for the initial RAM disk image, which
are picked up from the file system the kernel image is located on. Formatted as decimal ASCII string
(i.e. <literal>13</literal>). This variable is set if a measurement was successfully completed, and
remains unset otherwise.</para></listitem>
</varlistentry>
</variablelist>
<para>Note that some of the variables above may also be set by the boot loader. The stub will only set

4
src/boot/efi/measure.h
View File

@ -22,8 +22,8 @@
#define TPM_PCR_INDEX_KERNEL_PARAMETERS_COMPAT UINT32_MAX
#endif
/* This TPM PCR is where most Linux infrastructure extends the initrd binary images into, and so do we. */
#define TPM_PCR_INDEX_INITRD 4U
/* This TPM PCR is where we extend the initrd sysext images into which we pass to the booted kernel */
#define TPM_PCR_INDEX_INITRD_SYSEXTS 13U
#if ENABLE_TPM

29
src/boot/efi/stub.c
View File

@ -182,8 +182,8 @@ EFI_STATUS efi_main(EFI_HANDLE image, EFI_SYSTEM_TABLE *sys_table) {
char *cmdline = NULL;
_cleanup_free_ char *cmdline_owned = NULL;
int sections_measured = -1, parameters_measured = -1;
bool sysext_measured = false, m;
EFI_STATUS err;
bool m;
InitializeLib(image, sys_table);
debug_hook(L"systemd-stub");
@ -298,21 +298,24 @@ EFI_STATUS efi_main(EFI_HANDLE image, EFI_SYSTEM_TABLE *sys_table) {
&m) == EFI_SUCCESS)
parameters_measured = parameters_measured < 0 ? m : (parameters_measured && m);
(void) pack_cpio(loaded_image,
NULL,
L".raw",
".extra/sysext",
/* dir_mode= */ 0555,
/* access_mode= */ 0444,
/* tpm_pcr= */ (uint32_t[]) { TPM_PCR_INDEX_INITRD },
/* n_tpm_pcr= */ 1,
L"System extension initrd",
&sysext_initrd,
&sysext_initrd_size,
NULL);
if (pack_cpio(loaded_image,
NULL,
L".raw",
".extra/sysext",
/* dir_mode= */ 0555,
/* access_mode= */ 0444,
/* tpm_pcr= */ (uint32_t[]) { TPM_PCR_INDEX_INITRD_SYSEXTS },
/* n_tpm_pcr= */ 1,
L"System extension initrd",
&sysext_initrd,
&sysext_initrd_size,
&m) == EFI_SUCCESS)
sysext_measured = m;
if (parameters_measured > 0)
(void) efivar_set_uint_string(LOADER_GUID, L"StubPcrKernelParameters", TPM_PCR_INDEX_KERNEL_PARAMETERS, 0);
if (sysext_measured)
(void) efivar_set_uint_string(LOADER_GUID, L"StubPcrInitRDSysExts", TPM_PCR_INDEX_INITRD_SYSEXTS, 0);
linux_size = szs[SECTION_LINUX];
linux_base = POINTER_TO_PHYSICAL_ADDRESS(loaded_image->ImageBase) + addrs[SECTION_LINUX];