IMA: Measure kernel version in early boot

The integrity of a kernel can be verified by the boot loader on cold
boot, and during kexec, by the current running kernel, before it is
loaded. However, it is still possible that the new kernel being
loaded is older than the current kernel, and/or has known
vulnerabilities. Therefore, it is imperative that an attestation
service be able to verify the version of the kernel being loaded on
the client, from cold boot and subsequent kexec system calls,
ensuring that only kernels with versions known to be good are loaded.

Measure the kernel version using ima_measure_critical_data() early on
in the boot sequence, reducing the chances of known kernel
vulnerabilities being exploited. With IMA being part of the kernel,
this overall approach makes the measurement itself more trustworthy.

To enable measuring the kernel version "ima_policy=critical_data"
needs to be added to the kernel command line arguments.
For example,
        BOOT_IMAGE=/boot/vmlinuz-5.11.0-rc3+ root=UUID=fd643309-a5d2-4ed3-b10d-3c579a5fab2f ro nomodeset ima_policy=critical_data

If runtime measurement of the kernel version is ever needed, the
following should be added to /etc/ima/ima-policy:

        measure func=CRITICAL_DATA label=kernel_info

To extract the measured data after boot, the following command can be used:

        grep -m 1 "kernel_version" \
        /sys/kernel/security/integrity/ima/ascii_runtime_measurements

Sample output from the command above:

        10 a8297d408e9d5155728b619761d0dd4cedf5ef5f ima-buf
        sha256:5660e19945be0119bc19cbbf8d9c33a09935ab5d30dad48aa11f879c67d70988
        kernel_version 352e31312e302d7263332d31363138372d676564623634666537383234342d6469727479

The above hex-ascii string corresponds to the kernel version
(e.g. xxd -r -p):

        5.11.0-rc3-16187-gedb64fe78244-dirty

Signed-off-by: Raphael Gianotti <raphgi@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

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