linux-kernel - Re: [PATCH v3] IMA: Measure kernel version in early boot

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Date:   Tue, 26 Jan 2021 11:14:43 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Raphael Gianotti <raphgi@...ux.microsoft.com>
Cc:     linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        tusharsu@...ux.microsoft.com, nramas@...ux.microsoft.com,
        tyhicks@...ux.microsoft.com
Subject: Re: [PATCH v3] IMA: Measure kernel version in early boot

On Mon, 2021-01-25 at 16:50 -0800, Raphael Gianotti wrote:
> 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 corresponds to the following (decoded) version string:

Instead of the above, the following is clearer.

    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@...ux.microsoft.com>

Assuming the above or similar change,

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