This commit adds a new `systemd.credentials_boot_policy=` kernel
commandline that allows to control if credentials with
a `null` key are accepted.
The possible options are:
* strict: always insist on tpm encryption
* tofu: allow null encryption in firstboot mode and when no tpm is available
* relaxed: allow null encryption when sb is off, or no tpm is available
* off: allow null encryption always
The default is currently `relaxed` which is the same behavior
as before.
This replaces the initial idea of using plaintext credentials
at firstboot (thanks to Lennart for this nicer and simpler design).
Note that this also moves `in_first_boot()` to `basic/initrd-util`
which is a better fit now.
This reworkds TPM2 based creds a bit. Instead of mapping the key type
"tpm2" directly to a TPM2 key without PK, let's map it to an "automatic"
key type that either picks PK or doesn't, depending on what's available.
That should make things easier to grok for people, as the nitty gritty
details of PK or not PK are made autmatic. Moreover it gives us more
leverage to change the TPM2 enrollment types later (for example, we
definitely want to start pinning SRK, and hook up pcrlock too, for
creds, which we currently don't).
This hence adds a new _CRED_AUTO_TPM2
pseudo-type we automatically maps to CRED_AES256_GCM_BY_TPM2_HMAC_WITH_PK
or CRED_AES256_GCM_BY_TPM2_HMAC depending if PK as available. Similar,
_CRED_AUTO_HOST_AND_TPM2 is added, which does the same for the
host/nonhost cred type.
This does not introduce any new type on the wire, it just changes how we
select the right key type.
To make the code more readable this also adds some categorization macros
for the keys, instead of repeating the list of key types at multiple
places.
Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.
The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.
Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.
The ability to encrypt/authenticate encryption with a null key was
originally just a fallback concept for cases where during early boot we
have no host key, but the local system has no TPM2. Nowadays it is used
for other stuff as well, such as pcrlock data propagation (i.e. data
that needs no protection itself and required to properly to TPM key
derivation).
Let's give better, explicit control over null key usage, i.e. let's make
it a tristate both on the systemd-creds command line and in the Varlink
IPC to control three cases:
- the default that we allow it only if SecureBoot is off
- explicitly allowed
- explicitly refused (this is new)
Ideally systemd-creds --allow-null switch would take a boolean argument
to control this as a tristate. Alas, that would be a compat break, hence
I added --refuse-null instead (which also maps to the low-level flag for
this).
This also normalizes that the null key is always called "null key" in
messages, and not sometimes "empty key" or "fallback key".
So far credentials are a concept for system services only: to encrypt or
decrypt credential you must be privileged, as only then you can access
the TPM and the host key.
Let's break this up a bit: let's add a "user-scoped" credential, that
are specific to users. Internally this works by adding another step to
the acquisition of the symmetric encryption key for the credential: if a
"user-scoped" credential is used we'll generate an symmetric encryption
key K as usual, but then we'll use it to calculate
K' = HMAC(K, flags || uid || machine-id || username)
and then use the resulting K' as encryption key instead. This basically
includes the (public) user's identity in the encryption key, ensuring
that only if the right user credentials are specified the correct key
can be acquired.
This is what it is after all: encryption with a NULL key. This is more
descriptive, but also relevant since we want to use this kind of
credentials in a different context soon: for carrying pcrlock data into
a UKI. In that case we don#t want encryption, since the pcrlock data is
intended to help unlocking secrets, hence should not be a secret itself.
This only changes the code labels and the way this is labelled in the
output. We retain compat with the old name.
This is just like read_credential() but also looks into the encrypted
credential directory, not just the regular one.
Normally, we decrypt credentials at the moment we pass them to services.
From service PoV all credentials are hence decrypted credentials.
However, when we want to access credentials in a generator this logic
does not apply: here we have the regular and the encrypted credentials
directory. So far we didn't attempt to make use of credentials in
generators hence.
Let's address and add helper that looks into both directories, and talks
to the TPM if necessary to decrypt the credentials.
Let's restrict how we apply credential globbing in ImportCredential=, so
that we have some flexibility in automatically extending the glob
expression with per-instance data eventually without getting into
conflict with the globbing parts.
In our current uses we only allow globbing at the end of the expression,
and this is a new, unreleased feature hence let's be restrictive on this
initially. We can still relax this later if we feel the need to after
all.
Fixes: #28022
Let's beef up our system credential game a bit, and explicitly import
creds from sd-stub, from qemu fw_cfg and the kernel cmdline and expose
them in the same way as those passed in from nspawn.
Specifically, this will imprt such credentials to
/run/credentials/@system (if the source can be trusted, as in the
qemu/kernel cmdline case) and /run/credentials/@encrypted (otherwise,
such as sd-stub provided ones).
Once imported we'll set the $CREDENTIALS_PATH env var for PID 1, like it
would be done by a container manager for the payload. (Conversely, we'll
also creat a symlink from /run/credentials/@system to whatever is set in
$CREDENTIALS_PATH in case we are invoked by a container manager, thus
providing a fixed path where system credentials are found).
This is supposed to be useful when generating credentials for immutable
initrd environments, where it is is relevant to support credentials even
on systems lacking a TPM2 chip.
With this, if `systemd-creds encrypt --with-key=auto-initrd` is used a
credential will be encrypted/signed with the TPM2 if it is available and
recognized by the firmware. Otherwise it will be encrypted/signed with
the fixed empty key, thus providing no confidentiality or authenticity.
The idea is that distributions use this mode to generically create
credentials that are as locked down as possible on the specific
platform.
Previously, when encrypting creds you could pick which key to use for
this via a 128bit ID identifying the key type, and use an all zero ID
for rquesting automatic mode.
Let's change this to use an explicitly picked 128bit ID for automatic
mode, i.e. something other than all zeros. This is in preparation for
adding one further automatic mode with slightly different semantics.
no change in behaviour.
Note that the new 128bit id is never written to disk but only used
internally to indicate a specific case.
This is preparation for adding encryption support to the credentials
logic, and we thus would like to add more deps. Let's hence move things
from src/basic/ to src/shared, so that we can rely on the OpenSSL
utilities already in src/shared.
Michael Vogt
Mike Yuan
Lennart Poettering
Daan De Meyer
Yu Watanabe
Ludwig Nussel