ima: support fs-verity file digest based version 3 signatures

IMA may verify a file's integrity against a "good" value stored in the
'security.ima' xattr or as an appended signature, based on policy. When
the "good value" is stored in the xattr, the xattr may contain a file
hash or signature. In either case, the "good" value is preceded by a
header. The first byte of the xattr header indicates the type of data
- hash, signature - stored in the xattr. To support storing fs-verity
signatures in the 'security.ima' xattr requires further differentiating
the fs-verity signature from the existing IMA signature.

In addition the signatures stored in 'security.ima' xattr, need to be
disambiguated. Instead of directly signing the fs-verity digest, a new
signature format version 3 is defined as the hash of the ima_file_id
structure, which identifies the type of signature and the digest.

The IMA policy defines "which" files are to be measured, verified, and/or
audited. For those files being verified, the policy rules indicate "how"
the file should be verified. For example to require a file be signed,
the appraise policy rule must include the 'appraise_type' option.

appraise_type:= [imasig] | [imasig|modsig] | [sigv3]
where 'imasig' is the original or signature format v2 (default),
where 'modsig' is an appended signature,
where 'sigv3' is the signature format v3.

The policy rule must also indicate the type of digest, if not the IMA
default, by first specifying the digest type:

digest_type:= [verity]

The following policy rule requires fsverity signatures. The rule may be
constrained, for example based on a fsuuid or LSM label.

appraise func=BPRM_CHECK digest_type=verity appraise_type=sigv3

Acked-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: permit fsverity's file digests in the IMA measurement list

Permit fsverity's file digest (a hash of struct fsverity_descriptor) to
be included in the IMA measurement list, based on the new measurement
policy rule 'digest_type=verity' option.

To differentiate between a regular IMA file hash from an fsverity's
file digest, use the new d-ngv2 format field included in the ima-ngv2
template.

The following policy rule requires fsverity file digests and specifies
the new 'ima-ngv2' template, which contains the new 'd-ngv2' field. The
policy rule may be constrained, for example based on a fsuuid or LSM
label.

measure func=FILE_CHECK digest_type=verity template=ima-ngv2

Acked-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: define a new template field named 'd-ngv2' and templates

In preparation to differentiate between unsigned regular IMA file
hashes and fs-verity's file digests in the IMA measurement list,
define a new template field named 'd-ngv2'.

Also define two new templates named 'ima-ngv2' and 'ima-sigv2', which
include the new 'd-ngv2' field.

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: fix 'd-ng' comments and documentation

Initially the 'd-ng' template field did not prefix the digest with either
"md5" or "sha1" hash algorithms. Prior to being upstreamed this changed,
but the comments and documentation were not updated. Fix the comments
and documentation.

Fixes: 4d7aeee73f53 ("ima: define new template ima-ng and template fields d-ng and n-ng")
Reported-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

doc: Fix warning in Documentation/security/IMA-templates.rst

This patch fixes the warning:

Documentation/security/IMA-templates.rst:81: WARNING: Inline
substitution_reference start-string without end-string.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: Define new template evm-sig

With the recent introduction of the evmsig template field, remote verifiers
can obtain the EVM portable signature instead of the IMA signature, to
verify file metadata.

After introducing the new fields to include file metadata in the
measurement list, this patch finally defines the evm-sig template, whose
format is:

d-ng|n-ng|evmsig|xattrnames|xattrlengths|xattrvalues|iuid|igid|imode

xattrnames, xattrlengths and xattrvalues are populated only from defined
EVM protected xattrs, i.e. the ones that EVM considers to verify the
portable signature. xattrnames and xattrlengths are populated only if the
xattr is present.

xattrnames and xattrlengths are not necessary for verifying the EVM
portable signature, but they are included for completeness of information,
if a remote verifier wants to infer more from file metadata.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: Define new template fields xattrnames, xattrlengths and xattrvalues

This patch defines the new template fields xattrnames, xattrlengths and
xattrvalues, which contain respectively a list of xattr names (strings,
separated by |), lengths (u32, hex) and values (hex). If an xattr is not
present, the name and length are not displayed in the measurement list.

Reported-by: kernel test robot <lkp@intel.com> (Missing prototype def)
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: Define new template field imode

This patch defines the new template field imode, which includes the
inode mode. It can be used by a remote verifier to verify the EVM portable
signature, if it was included with the template fields sig or evmsig.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: Define new template fields iuid and igid

This patch defines the new template fields iuid and igid, which include
respectively the inode UID and GID. For idmapped mounts, still the original
UID and GID are provided.

These fields can be used to verify the EVM portable signature, if it was
included with the template fields sig or evmsig.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: Introduce template field evmsig and write to field sig as fallback

With the patch to accept EVM portable signatures when the
appraise_type=imasig requirement is specified in the policy, appraisal can
be successfully done even if the file does not have an IMA signature.

However, remote attestation would not see that a different signature type
was used, as only IMA signatures can be included in the measurement list.
This patch solves the issue by introducing the new template field 'evmsig'
to show EVM portable signatures and by including its value in the existing
field 'sig' if the IMA signature is not found.

Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

ima: Define ima-modsig template

Define new "d-modsig" template field which holds the digest that is
expected to match the one contained in the modsig, and also new "modsig"
template field which holds the appended file signature.

Add a new "ima-modsig" defined template descriptor with the new fields as
well as the ones from the "ima-sig" descriptor.

Change ima_store_measurement() to accept a struct modsig * argument so that
it can be passed along to the templates via struct ima_event_data.

Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

IMA: Define a new template field buf

A buffer(kexec boot command line arguments) measured into IMA
measuremnt list cannot be appraised, without already being
aware of the buffer contents. Since hashes are non-reversible,
raw buffer is needed for validation or regenerating hash for
appraisal/attestation.

Add support to store/read the buffer contents in HEX.
The kexec cmdline hash is stored in the "d-ng" field of the
template data. It can be verified using
sudo cat /sys/kernel/security/integrity/ima/ascii_runtime_measurements |
grep kexec-cmdline | cut -d' ' -f 6 | xxd -r -p | sha256sum

- Add two new fields to ima_event_data to hold the buf and
buf_len
- Add a new template field 'buf' to be used to store/read
the buffer data.
- Updated process_buffer_meaurement to add the buffer to
ima_event_data. process_buffer_measurement added in
"Define a new IMA hook to measure the boot command line
arguments"
- Add a new template policy name ima-buf to represent
'd-ng|n-ng|buf'

Signed-off-by: Prakhar Srivastava <prsriva02@gmail.com>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

doc: ReSTify IMA-templates.txt

Adjust IMA-templates.txt for ReST markup and add to the index for
security/, under the Kernel API Documentation.

Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>