Re: [PATCH v2] IMA: Measure kernel version in early boot

[Mimi Zohar <zohar@linux.ibm.com>]

On Fri, 2021-01-22 at 15:28 -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_version
> 
> 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:
> 
> 	5.11.0-rc3-16187-gedb64fe78244-dirty
> 
> This patch is based on
> commit e58bb688f2e4 "Merge branch 'measure-critical-data' into next-integrity"
> in "next-integrity-testing" branch
> 
> Change Log v2:
> 	- Changed the measurement to align with the latest version of
> 	  ima_measure_critical_data(), without the need for queueing
> 	- Scoped the measurement to only measure the kernel version,
> 	  found in UTS_RELEASE, instead of the entire linux_banner
> 	  string
> 
> Signed-off-by: Raphael Gianotti <raphgi@linux.microsoft.com>
> ---
>  security/integrity/ima/ima_main.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 6a429846f90a..0a33f570725c 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -26,6 +26,7 @@
>  #include <linux/ima.h>
>  #include <linux/iversion.h>
>  #include <linux/fs.h>
> +#include <generated/utsrelease.h>
>  
>  #include "ima.h"
>  
> @@ -994,8 +995,11 @@ static int __init init_ima(void)
>  	if (error)
>  		pr_warn("Couldn't register LSM notifier, error %d\n", error);
>  
> -	if (!error)
> +	if (!error) {
>  		ima_update_policy_flag();
> +		ima_measure_critical_data("kernel_version", "kernel_version",
> +					  UTS_RELEASE, strlen(UTS_RELEASE), false);
> +	}
>  
>  	return error;
>  }

Consider defining a new critical data label grouping (e.g.
"kernel_info",  ...).

Please move ima_measure_critical_data() to ima_init() and update the
critical data "label:=" in Documentation/ABI/testing/ima_policy.

thanks,

Mimi